First, a baseline assessment will take place. Magnetic resonance imaging of the brains will be performed to assess brain volume and magnetic resonance spectroscopy will be performed to measure the concentrations of specific substances in the brains. Changes in the volume of parts of the brains have been observed in (non-cancer) patients with the chronic fatigue syndrome (CFS), in comparison with healthy controls. In addition, abnormal concentrations of specific substances have been observed in patients with CFS compared to healthy controls. To assess muscle fatigue, a two-minute endurance test of the upper arm will be administered at maximal voluntary contraction. Next to differences in the brains, CFS patients showed (central) muscle fatigue. A maximal exercise test on a bicycle will be performed to assess physical fitness. Physical activity in fatigued cancer survivors is decreased, compared to healthy controls. It is not known whether physical deconditioning originated during the cancer treatment is the reason why these patients are still less active. In addition, patients and controls will wear an actometer for two weeks to register baseline daily physical activity and for an additional 5 days after the maximal exercise test, to assess the effect of exercise on the daily physical activity. Finally, patients and controls will complete standardized questionnaires and will perform neurological/psychological tests, like a reaction time test and a short time memory task, at baseline.