Although fatigue is one of the most common symptoms of multiple sclerosis, it is yet poorly understood and therefore difficult to manage. To clarify the nature of fatigue we investigated its relationship to depression, physical impairment, personality and action control and compared these variables between a sample of 41 MS patients and 41 healthy controls. Physical impairment was assessed by the EDSS and all other dimensions, using questionnaires. Stepwise linear regression analyses revealed that physical impairment was related to physical fatigue in MS patients. Depression was the main factor influencing fatigue among both, MS patients and controls. What clearly differentiated the two groups was the correlation between fatigue and action control. Decreased levels of action control imply attentional and motivational deficits and were only found in fatigued MS patients. Our study indicates that motivational disturbances might be specific for MS related fatigue.