Twenty-six women with primary fibromyalgia were tested for muscle strength and for aerobic capacity and fatigue. Grip strength was measured with a manometer. In the patient group the grip strength was 58 +/- 22 kPa (mean +/- SD) versus 97 +/- 17 kPa in healthy matched controls (p less than 0.01). The patients performed a mean of 10 +/- 6 repetitions with maximal grip pressure (dynamic endurance work), while the controls performed 13 +/- 6 repetitions (p less than 0.01). The upper extremity was kept in a fixed position for 152 +/- 147 seconds in the patient group versus 413 +/- 244 seconds in the control group (static endurance work), p less than 0.004. Normal aerobic capacity (O2 ml/kg min) was found in the patient group. Expected fatigue values were determined by Borg's rating scale for perceived exertion according to the heart rate in a cycle ergometer test. There was a significantly higher mean fatigue score in the patient group as compared to the expected values (p less than 0.0001).