In 119 patients referred with suspected fibromyalgia, biopsies from the quadriceps muscle were analyzed for "rubber band" morphology, and isokinetic quadriceps strength was measured. Eighty-four fulfilled the criteria for fibromyalgia, 26 had chronic myofascial pain (CMP) and 9 had other diseases including 5 with concomitant fibromyalgia. Twenty-four CMP patients and 48 fibromyalgia patients were randomly selected to match with regard to sex, age, smoking and drinking habits. "Rubber band" morphology was blindly graded on a biopsy score scale from 0 to 2. A statistically significant difference in biopsy score was found between the two matched groups (P = 0.003); median biopsy score in fibromyalgia was 0.42 and 0.25 in CMP. A cut-off value at 0.33 gave a specificity of 71% and a sensitivity of 63%. Isokinetic muscle strength did not differ in the fibromyalgia and CMP groups and was not related to the biopsy score. "Rubber band" morphology is seen more often in fibromyalgia patients than in CMP patients. The exact genesis of this phenomenon is still unknown but theories connected with the possible pathogenesis of the syndrome are presented.