We studied 100 multiple sclerosis outpatients in order to assess the frequency of fatigue and to detect its relationships with other clinical findings. Fatigue was quantified by fatigue severity scale (FSS; range 1-7); current clinical neurological status and disability level were respectively scored by Kurtzke's functional systems and expanded disability status scale (EDSS). FSS mean score was 4.12; fatigue was totally absent in 3% of the patients. Fatigue started before onset of the disease in 7 patients, at onset in 12 and within the first year in 7. Fatigue intensity and frequency were related to each other. FSS mean scores were significantly higher in the patients with fatigue worsened by heat (p < 0.01), with chronic progressive disease (p < 0.0001), with motor symptoms (p < 0.0001) and with EDSS > or = 3.5 (p < 0.0001). Multiple regression analysis showed a significant effect of EDSS on fatigue (p < 0.0001), not attributable to differences in duration of the disease.