Sonography of the popliteal space was performed in 28 patients with a history of a popliteal mass, verified by physical examination in all but 7 patients. The final diagnosis was established by needle biopsy or surgery. Three main types of lesions were found sonographically. A thin-walled echo-free cyst was present in 16 patients. The diagnosis in all of these cases was Baker's cyst. Two patients had a mixed cystic and solid lesion, which was found to be a Baker's cyst, with a loose body in 1 case and a soft-tissue sarcoma with hematoma in the other. Three patients showed solid lesions subsequently diagnosed as an old rupture of the semimembranosus muscle, lipoma, and soft tissue sarcoma. Sonography was normal in the 7 patients without clinical evidence of a popliteal mass. Sonography of popliteal masses is useful to distinguish benign cysts from other lesions.