We assessed muscle pathology in 30 patients with ankylosing spondylitis (AS) and 22 controls to assess if skeletal muscle is affected primarily by the inflammatory process of the disease. Investigations included a questionnaire on musculoskeletal discomfort, physical exercises, dynamometric measurements, EMG, and biopsy of the quadriceps muscle. Symptoms of muscular weakness were related with enthesopathic activity index. Plasma CK was higher in patients than in controls. A myopathic EMG pattern was found in 46.4% patients. Histological changes were found in 66% and did not correlate with symptomatology. Patients with AS with clinical muscular manifestations probably have intense enthesopathic inflammatory activity. It is suggested that muscles are secondarily affected as a consequence of pain inhibition and reduced activity.