Aerobic fitness was evaluated in 25 women with fibrositis, by having them exercise to volitional exhaustion on an electronically braked cycle ergometer. Compared with published standards, >80% of the fibrositis patients were not physically fit, as assessed by maximal oxygen uptake. Compared with matched sedentary controls, fibrositis patients accurately perceived their level of exertion in relation to oxygen consumption and attained a similar level of lactic acidosis, as assessed by their respiratory quotient and ventilatory threshold. Exercising muscle blood flow was estimated by ¹³³xenon clearance in a subgroup of 16 fibrositis patients and compared with that in 16 matched sedentary controls; the fibrositis patients exhibited reduced ¹³³xenon clearance. These results indicate a need to include aerobic fitness as a matched variable in future controlled studies of fibrositis and suggest that the “detraining phenomenon” may be of relevance to the etiopathogenesis of the disease.