The objective of the study was to evaluate the physical capacity and effort in patients with fibromyalgia. Muscle strength and the coefficient of variation of the strength measurements of 181 female fibromyalgia patients and 126 healthy females were compared. These measurements and ergometer exercise capacity, work status and psychometric scoring (SCL-90-R) were correlated. The fibromyalgia patients exhibited significant reduction in voluntary muscle strength of the knee and elbow, flexors and extensors in the order of 20-30%. However, the coefficient of variation was higher among patients, thus indicating lower effort. The physical performance during an ergometer test corresponded to a maximal oxygen consumption of 21 ml/kg-1 x min-1. The maximal increase in heart rate was only 63% (44-90%) of the predicted increase. Degree of effort or physical capacity did not correlate to psychometric scores. Work status was related to psychometric scoring, but not to physical capacity or effort. In conclusion, we found a low degree of effort but near normal physical capacity in the fibromyalgia patients.