In thirty cases of synovial osteochondromatosis studied clinically and pathologically, the process appeared to follow a temporal sequence characterized by three recognizable phases (1) active intrasynovial disease only, with no loose bodies; (2) transitional lesions with both active intrasynovial proliferation and free loose bodies; and (3) multiple free osteochondral bodies with no demonstrable intrasynovial disease. If gross examination at the time of surgery shows that the disease is in the third phase, it would appear that synovectomy may not be necessary. However, extrasynovial intra-articular cartilaginous lesions may persist after synovectomy in the other two phases and may grow, causing recurrence of clinical symptoms in the absence of new intrasynovial disease.