Not the most intuitive error message, I must say, but that has resolved that particular issue. Thanks Marcel.

I've put the QApplication include at the top of main.cpp, but now I get the following compiler error:

mingw32-make -f Makefile.Debug
mingw32-make[1]: Entering directory `C:/projects/primecalc'
g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_
THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\Qt\4.3.0\include\QtCore" -I"..\..\Qt\4.3.0\include\QtCore" -I"..\..\Qt\4.3.0\in
clude\QtGui" -I"..\..\Qt\4.3.0\include\QtGui" -I"..\..\Qt\4.3.0\include" -I"." -I"..\..\Qt\4.3.0\include\ActiveQt" -I"de
bug" -I"." -I"..\..\Qt\4.3.0\mkspecs\win32-g++" -o debug\main.o main.cpp
main.cpp:5: error: new types may not be defined in a return type
main.cpp:5: error: extraneous `int' ignored
main.cpp: In function `PrimeDialog qMain(int, char**)':
main.cpp:9: error: conversion from `int' to non-scalar type `PrimeDialog' requested
mingw32-make[1]: *** [debug\main.o] Error 1
mingw32-make[1]: Leaving directory `C:/projects/primecalc'
mingw32-make: *** [debug] Error 2

Once again, any ideas are appreciated. I don't see how I'm trying to convert an int to my PrimeDialog class.

Jimmy