We describe a patient with acquired immunodeficiency syndrome who developed Staphylococcus aureus septic arthritis and osteomyelitis of the acromioclavicular joint. The case is unusual because of the rarity of reported bone and joint infections in patients with acquired immunodeficiency syndrome and because of the indolent nature of the infection.Patients with acquired immunodeficiency syndrome (AIDS) may develop a variety of systemic and localized infections. Frequently, these infections have clinical features that are rarely seen in immunocompetent hosts. Bone and joint infections in patients with AIDS have been reported only rarely. There are a few case reports of fungal septic arthritis (1,2) and osteomyelitis (3,4) in these patients; however, we are aware of no previous reports of well-documented bacterial septic arthritis in AIDS patients. We describe a patient with AIDS who developed septic arthritis and osteomyelitis involving the acromioclavicular (AC) joint. A review of the literature and discussion of treatment are presented. Case report. The patient, a homosexual man, was diagnosed as having AIDS in August 1986 at the age of 25. There was no history of intravenous drug abuse. He experienced 2 episodes of Pneumocystis carinii pneumonia, but was doing well on a regimen of zidovudine (azidothymidine; AZT) until January 1988, when a dull ache developed in his right shoulder. The shoulder pain initially improved with nonsteroidal antiinflammatory drug treatment. Four months later, he again reported having shoulder pain, especially while playing tennis or lifting weights, and he noticed warm swelling in the area of the AC joint. He denied having fever, chills, or constitutional symptoms.There had been no peripheral intravenous lines, indwelling catheters, or other skin invasion near the AC joint.In August 1988, a bronchoscopy and gallium scan were performed to evaluate a persistent, productive cough. There was increased uptake of the gallium tracer in the distal end of the right clavicle and in the acromion, suggestive of osteomyelitis. He was referred to the rheumatology service. On physical examination, the vital signs were normal, and there was no evidence of skin rash, heart murmur, or systemic illness. The lungs were clear to auscultation. Musculoskeletal examination results were remarkable only for warm, tender swelling anterior to the AC joint. There was normal range of motion of the shoulder, but adduction across the chest produced pain in the anterior shoulder region.Laboratory evaluation revealed a white blood cell count of 6,400/mm3 (12.2% lymphocytes, 9.4% monocytes, 78% granulocytes), a hemoglobin level of 13.8 gm/dl, and a hematocrit value of 39.5%. Attempted aspiration of the right AC joint yielded < 1 cc