Individuals with sickle-cell disease may develop gout, septic arthritis, osteomyelitis, aseptic osteonecrosis, and possibly hemarthrosis. Other patients have only arthralgia or joint effusions. These effusions, studied here for the first time, are "noninflammatory," despite the frequent acute onset, warmth, and tenderness that might clinically suggest other types of arthritis. Joint effusions most often occurred with other evidences of painful crises. Synovial biopsies in five patients, aged 6 to 29 years, showed microvascular thrombosis that, along with vascular occlusion in adjacent tissues, may be the mechanism for the effusions. Sickled erythrocytes in effusions can suggest sickle-cell arthropathy. Adjacent bones may show evidence of previous infarctions.