To study the characteristic diagnostic imaging features of idiopathic synovial osteochondromatosis with different imaging techniques as well as the differential diagnosis between idiopathic and secondary forms, 7 patients with idiopathic synovial osteochondromatosis were retrospectively examined from January, 1988, through December, 1993. In 6 patients the disease was monoarticular (the shoulder in 4 patients, the elbow in one patient and the hip in another one). In one patient two joints were involved--i.e., the left elbow and the right knee. In all cases the clinical picture was characterized by pain and functional impairment of the involved joint, which was swollen in 3 patients. The definitive diagnosis was made on the basis of surgical and/or arthroscopic findings in 4 cases and of radiologic and MR findings in 4 cases and of radiologic and MR findings in 4 cases. In all patients conventional radiology showed multiple juxtaarticular calcified and/or ossified bodies, mostly uniform in size. Calcification patterns varied from small specks of calcification (2-3 mm) to large calcified bodies (1-2 cm) with a typical "cluster" appearance. Conventional arthrography and computerized arthrotomography (performed on 3 patients) demonstrated the intraarticular site of calcified bodies as filling defects at hypertrophic synovial recesses. CT and MRI (performed on 4 patients) allowed easier demonstration of calcified/ossified bodies and of their relationships with articular structures. Moreover, synovial proliferation and cartilagineous nodules obliterating articular spaces, synovial recesses and juxtaarticular bursae were depicted. In particular, MRI allowed the differentiation not only of the different stages of the disease, but also of the idiopathic from the secondary forms. MRI plays therefore a fundamental role in the diagnosis of idiopathic synovial osteochondromatosis, directly influencing surgical planning, i.e., removing articular loose bodies with partial or total synoviectomy by means of arthroscopy.