Fatigue after curative treatment for cancer is a severe and invalidating problem. 20-40% of disease-free cancer patients mention fatigue as a frequent complaint, impairing quality of life. In search for (neuro)physiological factors determining fatigue, our centre has recently demonstrated the presence of morphological differences in the brains of non-cancer patients with the chronic fatigue syndrome (CFS) compared with healthy volunteers. Both in patients with CFS and in fatigued patients with neuromuscular diseases we showed that fatigue has a central neurophysiological component (so-called central activation failure). Others have shown that chronic fatigue is associated with altered brain metabolism. Studies with proton MR spectroscopy (1H MRS) have demonstrated a higher choline to creatine ratio in the brains of chronically fatigued patients. This suggests increased cell membrane turnover. Also reduced levels of N-acetylaspartate-creatine ratio (NAA/Cr) in the right hippocampus have been observed in these patients, which suggests a decrease in functional axons. Finally, elevated ventricular lactate was observed, which suggests changes in brain glucose metabolism. Actigraphy has shown that actual physical activity in fatigued cancer survivors is decreased compared to healthy controls. It is not known whether physical deconditioning originated during the actual cancer treatment is the reason why these patients are still less active. Until now no other (neuro)physiological factors have been identified explaining fatigue in cancer survivors. Recently we have shown that Cognitive Behaviour Therapy (CBT) especially designed for fatigued cancer patients is an effective treatment.