Physical fatigability and avoidance of physically demanding tasks in chronic fatigue syndrome (CFS) were assessed by the achievement or nonachievement of 85% of age-predicted maximal heart rate (target heart rate, THR) during incremental exercise. The association with functional status impairment, somatization, and psychopathology was examined. A statistically significant association was demonstrated between this physical fatigability variable and impairment, and a trend was found for an association with somatization. No association was demonstrated with psychopathology. These results are in accordance with the cognitive-behavioral model of CFS, suggesting a major contribution of avoidance behavior to functional status impairment; however, neither anxiety nor depression seem to be involved in the avoidance behavior. Aerobic work capacity was compared between CFS and healthy controls achieving THR. Physical deconditioning with early involvement of anaerobic metabolism was demonstrated in this CFS subgroup. Half of the CFS patients who did not achieve THR did not reach the anaerobic threshold. This finding argues against an association in CFS between avoidance of physically demanding tasks and early anaerobic metabolism during effort.