The authors present a case of primary synovial osteochondromatosis of the ankle, as demonstrated by magnetic resonance (MR). Ankle involvement by osteochondromatosis is unusual. The characteristic MR findings are: target appearance of some loose bodies with a low intensity peripheral rim and a center isointense to the soft tissues on T1-weighted images, which remain unchanged on T2-weighted images. Conversely, other loose bodies were characterized by homogeneous hyperintensity close to the bone marrow on T1-weighted images, with loss of signal on T2-weighted images. The authors believe that these findings are quite pathognomonic of a long-standing synovial osteochondromatosis.