A large overlap exists between the diagnosis of chronic fatigue syndrome (CFS) and the unexplained symptoms reported by many Gulf War veterans (GV). Previous investigations have reported reduced aerobic capacity in civilians with CFS. The present investigation examined metabolic responses to maximal exercise in GVs with CFS compared with healthy GVs. Cardiorespiratory and metabolic responses were recorded during a maximal exercise test on a cycle ergometer. The groups were not different in any demographic category (p > 0.05) or self-reported physical activity (p > 0.05). No differences were observed between groups for maximal oxygen uptake (28.9 +/- 6.7 mL/kg/min for CFS vs. 30.8 +/- 7.1 mL/kg/min for controls; p = 0.39), heart rate (155.8 +/- 16.1 bpm for CFS vs. 163.3 +/- 14.9 bpm for controls; p = 0.17), exercise time (9.6 +/- 1.5 minutes for CFS vs. 10.2 +/- 1.4 minutes for controls; p = 0.26), or workload achieved (208 +/- 36.7 W for CFS vs. 224 +/- 42.9 W for controls; p = 0.25). Likewise, no differences were observed at submaximal intensities (p > 0.05). Compared with healthy controls, GVs who report multiple medically unexplained symptoms and meet criteria for CFS do not show a decreased exercise capacity. Thus, it does not appear that the pathology of the GVs with CFS includes a deficiency with mobilizing the cardiopulmonary system for strenuous physical effort.