Tropical pyomyositis is an infection of large muscle groups that can lead to sepsis and death. The most common etiologic agent is Staphylococcus aureus. It usually occurs in patients living in the tropics but is seen with increasing frequency in temperate climates, particularly in immunosuppressed patients, where it may be misdiagnosed and may cause severe morbidity and mortality. Diagnosis is based on the examination of pus from a muscle aspirate and treatment consists of surgical incision, drainage and appropriate antibiotic therapy. It is stressed to take into account pyomyositis in the differential diagnosis of immunocompromised patients with "cryptic" myalgia.