Postcancer fatigue is a severe and invalidating problem, impairing quality of life. About 20 to 40% of the patients remain fatigued, at least one year after successful cancer treatment. Fortunately, there is an effective treatment for postcancer fatigue; cognitive behavior therapy. However, no cause for postcancer fatigue has been identified yet. The aim of the study is to identify factors that (partly) cause postcancer fatigue to improve the theoretical understanding of fatigue and to improve the diagnostics of fatigue, predict therapy outcome, and facilitate other treatment options.