The objective of the study was to examine the muscle performance, isokinetic muscle strength, muscle endurance ratio, and submaximal aerobic performance in fibromyalgia syndrome (FMS) patients, to evaluate the relation between muscle performance, pain severity, clinical findings, and physical activity level, and to compare the results with healthy control subjects. Twenty-four FMS patients and 15 control subjects participated in this study. Data were obtained about the symptoms, location and onset of pain, treatment, and associated symptoms. Patients and controls underwent an examination of isokinetic muscle strength of right quadriceps on a Cybex dynamometer, and submaximal aerobic performance tests (PWC-170) were done for all subjects. Maximal voluntary muscle strength of the quadriceps was significantly lower in patients compared with the control group. Endurance ratios showing the work capacity were not statistically different between two groups. Submaximal aerobic performance scores were higher in the control group. There was not a relation between the decreased muscle performance and clinical findings, including pain severity, number of tender points, and duration of the symptoms of FMS patients. We found a reduced quadriceps muscle and submaximal aerobic performance in patients with FMS, indicating that patients have impaired muscle function.