A subacute, oligoarthritic syndrome developed in four patients with human immunodeficiency virus (HIV) infection. Three had true acquired immunodeficiency syndrome (AIDS) and all had lymphocyte abnormalities. The arthritis was characterized by extreme pain and disability in three patients and moderate pain in one. Knees and ankles were affected. Symptoms developed over a one- to six-week interval; response to treatment was rapid, especially to intra-articular corticosteroids. Despite the clinical severity of the arthritis, synovial fluids were non-inflammatory and biopsy specimens revealed only mild chronic synovitis. A definite etiology could not be established. None of the patients had recognized infections predisposing to reactive arthritis, and the three patients who underwent tissue typing were HLA-B27-negative. A viral infection, including HIV, is a possible cause. In distinction to these four patients, arthritides with clearly established etiologies developed during this same time period in four other HIV-infected patients.