A brief mental fatigue questionnaire was administered to normal subjects and muscle-diseased, Chronic Fatigue Syndrome (CFS), recovered CFS and depressed patients. The questionnaire was found to have excellent internal consistency and discriminated effectively between CFS and depressed patients on the one hand and recovered CFS, normal and muscle-diseased patients on the other. However, the scale failed to discriminate between CFS and depressed subjects, who were found to experience qualitatively and quantitatively similar mental fatigue symptoms.