Question: I have downloaded qt-5.0.0 for windows. 

<http://releases.qt-project.org/qt5/5.0.1/qt-windows-opensource-5.0.1-msvc2010_32-x86-offline.exe>

I have added INCLUDEPATH += C:\opencv\build\include in the .pro file.

Opencv 2.4.3 is already installed.

When I include header file in qtcreator :

```
#include <opencv2/opencv.hpp> 

```

There is compilation error : can not find opencv2/opencv.hpp

Any ideas ??

Answer: 
Things to check:

* does `C:\opencv\build\include\opencv2\opencv.hpp` actually exist?
* does the compile command (which you can check in the Qt Creator "compile output" window) show `-IC:\opencv\build\include` argument in the compile command?
* qmake should be run automatically after modifying the .pro file, but re-run it manually just in case (for example from Build menu), as suggested by the first answer
* this should not have any effect in issue like this, but just in case: if you are using "shadow build" (which is a good idea), make sure the source dir is clean of any generated files
Well, I just ran into this problem tonight. luckily enough, after taking a fair time, the solution was found. If your project is managed by qmake, and Qcreator is used, just go to the `Build->Run qmake`, then build and run your project. A tip, whenever you change your `*.pro` file, remember to rerun `Build->Run qmake`, because that will reconfigure your project.
If this helps you, please give me a thumb up :)