The diagnosis of pyomyositis is sometimes difficult, because of its vague symptoms and relative rare incidences in temperate climates. We report a case of acute pyogenic infection in the quadratus femoris muscle, which simulated an acute septic arthritis of hip. Computerized tomography was useful in diagnosis. It was caused by Staphylococcus aureus. The patient recovered, fully, by surgical drainage and intravenous antibiotic therapy. Despite rare incidences, an acute pyomyositis should be considered in the differential diagnosis of pain in the hip region.