We describe a patient who developed acquired immunodeficiency syndrome-related complex, complicated by chronic, symmetric polyarthritis. Synovial biopsy showed large areas of plasma cell infiltration subjacent to the synovial lining. Ultrastructural study demonstrated tubuloreticular structures within endothelial cells, crystal-like inclusions in plasma cells, and virus-like particles located around synoviocyte fragments. Although immunologic and morphologic studies did not permit classification of these virus-like structures, the role of these possible virions in the pathogenesis of the observed synovitis remains to be determined. Surprisingly, the patient's chronic arthritis resolved with anti-retroviral treatment (azidothymidine: AZT).