Degree of premorbid 'action-proneness' was measured, using a self-administered questionnaire, in 35 patients suffering from chronic fatigue syndrome (CFS), all the members of 'ME'-self help groups and all those meeting CDC-criteria of CFS. The results were compared with those of 30 chronic idiopathic musculoskeletal pain patients, 34 patients with a chronic organic condition, and 34 neurotic patients without primary somatic complaints. Statistical analysis showed that CFS patients described themselves as significantly more 'action-prone' than the last two groups, and to a degree which was comparable with the chronic pain group. The results could not be explained by concomitant depression and are in accordance with anecdotal reports of premorbid hyperactive lifestyle in CFS patients. Further investigations seem worthwhile to test the hypothesis that hyperactivity might be a predisposing factor for chronic illness behaviour in CFS patients.