Muscle biopsies of the descending portion of the trapezius muscle from female patients with chronic trapezius myalgia and from healthy women were analyzed with enzyme histochemical and immunohistochemical methods. Frequency, area, and capillarization of the muscle fiber types were determined. A biochemical analysis determined the lactate concentration of mixed muscle samples and the adenosine triphosphate (ATP) and phosphocreatine levels in single muscle fibers. The patients had larger type I fibers and a lower capillary:fiber area ratio for type I and type IIA fibers. The patients also exhibited lower levels of ATP and phosphocreatine in both type I and type II fibers. It is suggested that there might have been an imbalance between the capillary supply and the cross-sectional fiber area of type I and type IIA fibers in the patients. This imbalance might be of significance in the development of muscular fatigue and pain.