diff --git a/include/wx/qt/msgdlg.h b/include/wx/qt/msgdlg.h
index c4b718696d..43616817bd 100644
--- a/include/wx/qt/msgdlg.h
+++ b/include/wx/qt/msgdlg.h
@@ -10,8 +10,6 @@
 
 #include "wx/msgdlg.h"
 
-class QMessageBox;
-
 class WXDLLIMPEXP_CORE wxMessageDialog : public wxMessageDialogBase
 {
 public:
diff --git a/src/qt/msgdlg.cpp b/src/qt/msgdlg.cpp
index 22251b0533..cade4e8010 100644
--- a/src/qt/msgdlg.cpp
+++ b/src/qt/msgdlg.cpp
@@ -14,13 +14,26 @@
 #include "wx/qt/private/winevent.h"
 
 #include <QtWidgets/qmessagebox.h>
+#include <QtWidgets/qpushbutton.h>
 
 
 class wxQtMessageDialog : public wxQtEventSignalHandler< QMessageBox, wxMessageDialog >
 {
+public:
+    wxQtMessageDialog( wxWindow *parent, wxMessageDialog *handler, const wxPoint& pos );
 
-    public:
-        wxQtMessageDialog( wxWindow *parent, wxMessageDialog *handler );
+    // Initialize the dialog to match the state of the associated wxMessageDialog.
+    //
+    // Does nothing if it had been already initialized.
+    void InitializeIfNeeded(wxMessageDialog& msgdlg);
+
+private:
+    // Initial position, may be wxDefaultPosition.
+    wxPoint m_pos;
+
+    // Set to true once the dialog has been initialized, which happens before
+    // it's shown for the first time.
+    bool m_initialized = false;
 };
 
 
@@ -28,90 +41,10 @@ wxMessageDialog::wxMessageDialog( wxWindow *parent, const wxString& message,
         const wxString& caption, long style, const wxPoint& pos )
     : wxMessageDialogBase( parent, message, caption, style )
 {
-    wxQtMessageDialog *dlg = new wxQtMessageDialog( parent, this );
-    m_qtWindow = dlg;
+    m_qtWindow = new wxQtMessageDialog( parent, this, pos );
 
-    // Set properties
-    Move( pos );
-    dlg->setText( wxQtConvertString( message ) );
-    dlg->setWindowTitle( wxQtConvertString( caption ) );
-
-    // Apply the style
-    SetWindowStyleFlag( style );
-
-    // Buttons
-    if ( style & wxOK )
-        dlg->addButton( QMessageBox::Ok );
-    if ( style & wxCANCEL )
-        dlg->addButton( QMessageBox::Cancel );
-    if ( style & wxYES_NO )
-    {
-        dlg->addButton( QMessageBox::Yes );
-        dlg->addButton( QMessageBox::No );
-    }
-
-    // Default button
-    if ( style & wxNO_DEFAULT )
-        dlg->setDefaultButton( QMessageBox::No );
-    else if ( style & wxCANCEL_DEFAULT )
-        dlg->setDefaultButton( QMessageBox::Cancel );
-    else
-    {
-        // Default to OK or Yes
-        if ( style & wxOK )
-            dlg->setDefaultButton( QMessageBox::Ok );
-        else
-            dlg->setDefaultButton( QMessageBox::Yes );
-    }
-
-    // Icon
-    int numIcons = 0;
-    if ( style & wxICON_NONE )
-    {
-        numIcons++;
-        dlg->setIcon( QMessageBox::NoIcon );
-    }
-
-    if ( style & wxICON_EXCLAMATION )
-    {
-        numIcons++;
-        dlg->setIcon( QMessageBox::Warning );
-    }
-
-    if ( style & wxICON_ERROR )
-    {
-        numIcons++;
-        dlg->setIcon( QMessageBox::Critical );
-    }
-
-    if ( style & wxICON_QUESTION )
-    {
-        numIcons++;
-        dlg->setIcon( QMessageBox::Question );
-    }
-
-    if ( style & wxICON_INFORMATION )
-    {
-        numIcons++;
-        dlg->setIcon( QMessageBox::Information );
-    }
-
-    wxCHECK_RET( numIcons <= 1, "Multiple icon definitions" );
-    if ( numIcons == 0 )
-    {
-        // Use default
-        if ( style & wxYES_NO )
-            dlg->setIcon( QMessageBox::Question );
-        else
-            dlg->setIcon( QMessageBox::Information );
-    }
-
-    if ( style & wxSTAY_ON_TOP )
-        dlg->setWindowModality( Qt::ApplicationModal );
-
-    PostCreation(false);
-
-    Centre(wxBOTH | wxCENTER_FRAME);
+    // The rest of the initialization will be done in InitializeIfNeeded()
+    // called from ShowModal().
 }
 
 wxIMPLEMENT_CLASS(wxMessageDialog,wxDialog);
@@ -121,8 +54,11 @@ int wxMessageDialog::ShowModal()
     WX_HOOK_MODAL_DIALOG();
     wxCHECK_MSG( m_qtWindow, -1, "Invalid dialog" );
 
+    auto dlg = static_cast<wxQtMessageDialog*>(m_qtWindow);
+    dlg->InitializeIfNeeded(*this);
+
     // Exec may return a wx identifier if a close event is generated
-    int ret = static_cast<QDialog*>(m_qtWindow)->exec();
+    int ret = dlg->exec();
     switch ( ret )
     {
         case QMessageBox::Ok:
@@ -145,7 +81,106 @@ wxMessageDialog::~wxMessageDialog()
 
 //=============================================================================
 
-wxQtMessageDialog::wxQtMessageDialog( wxWindow *parent, wxMessageDialog *handler )
-    : wxQtEventSignalHandler< QMessageBox, wxMessageDialog >( parent, handler )
+wxQtMessageDialog::wxQtMessageDialog(wxWindow *parent,
+                                     wxMessageDialog *handler,
+                                     const wxPoint& pos)
+    : wxQtEventSignalHandler< QMessageBox, wxMessageDialog >( parent, handler ),
+      m_pos(pos)
 {
 }
+
+void wxQtMessageDialog::InitializeIfNeeded(wxMessageDialog& msgdlg)
+{
+    setText( wxQtConvertString( msgdlg.GetMessage() ) );
+    setWindowTitle( wxQtConvertString( msgdlg.GetTitle() ) );
+
+    const wxString& extendedMessage = msgdlg.GetExtendedMessage();
+    if ( !extendedMessage.empty() )
+    {
+        setInformativeText( wxQtConvertString( extendedMessage ) );
+    }
+
+    // Buttons
+    const long style = msgdlg.GetMessageDialogStyle();
+
+    const auto applyCustomLabelIfSet = [](QPushButton* button, const wxString& label)
+    {
+        if ( !label.empty() )
+            button->setText( wxQtConvertString( label ) );
+    };
+
+    if ( style & wxOK )
+        applyCustomLabelIfSet(addButton( QMessageBox::Ok ), msgdlg.GetOKLabel());
+    if ( style & wxCANCEL )
+        applyCustomLabelIfSet(addButton( QMessageBox::Cancel ), msgdlg.GetCancelLabel());
+    if ( style & wxYES_NO )
+    {
+        applyCustomLabelIfSet(addButton( QMessageBox::Yes ), msgdlg.GetYesLabel());
+        applyCustomLabelIfSet(addButton( QMessageBox::No ), msgdlg.GetNoLabel());
+    }
+
+    // Default button
+    if ( style & wxNO_DEFAULT )
+        setDefaultButton( QMessageBox::No );
+    else if ( style & wxCANCEL_DEFAULT )
+        setDefaultButton( QMessageBox::Cancel );
+    else
+    {
+        // Default to OK or Yes
+        if ( style & wxOK )
+            setDefaultButton( QMessageBox::Ok );
+        else
+            setDefaultButton( QMessageBox::Yes );
+    }
+
+    // Icon
+    int numIcons = 0;
+    if ( style & wxICON_NONE )
+    {
+        numIcons++;
+        setIcon( QMessageBox::NoIcon );
+    }
+
+    if ( style & wxICON_EXCLAMATION )
+    {
+        numIcons++;
+        setIcon( QMessageBox::Warning );
+    }
+
+    if ( style & wxICON_ERROR )
+    {
+        numIcons++;
+        setIcon( QMessageBox::Critical );
+    }
+
+    if ( style & wxICON_QUESTION )
+    {
+        numIcons++;
+        setIcon( QMessageBox::Question );
+    }
+
+    if ( style & wxICON_INFORMATION )
+    {
+        numIcons++;
+        setIcon( QMessageBox::Information );
+    }
+
+    wxCHECK_RET( numIcons <= 1, "Multiple icon definitions" );
+    if ( numIcons == 0 )
+    {
+        // Use default
+        if ( style & wxYES_NO )
+            setIcon( QMessageBox::Question );
+        else
+            setIcon( QMessageBox::Information );
+    }
+
+    if ( msgdlg.HasFlag(wxSTAY_ON_TOP) )
+        setWindowModality( Qt::ApplicationModal );
+
+    // Now that the dialog has its size, we can position it.
+    if ( m_pos != wxDefaultPosition )
+        msgdlg.Move( m_pos );
+    else
+        msgdlg.Centre(wxBOTH | wxCENTER_FRAME);
+}