In this report, we have described three patients who had hemarthroses, with sickled red blood cells discovered by analysis of synovial fluid. On the basis of this observation, each patient was evaluated for the presence of abnormal hemoglobins, and each was found to have a hemoglobinopathy that was previously unsuspected. These patients differ from those in other reports in that two of the three had no associated arthritic condition that could readily explain synovitis or a condition that predisposed them to bleeding into a joint. Although the accumulated evidence suggests that heterozygous hemoglobinopathies do not produce arthritic syndromes, these reports again raise that question. We cannot conclude, however, that the hemarthroses were definitively caused by the underlying hematologic abnormality. Important when synovial fluid is mixed with blood, since other medical conditions can be diagnosed if abnormal findings are detected.