A 50-year-old woman with juvenile rheumatoid arthritis presented with painful swelling of the right calf. Roentgenograms, sonograms, magnetic resonance images, and arthrograms were obtained. A diagnosis of hemorrhagic Baker's cyst was made, and surgical excision was performed. The etiology, clinical features, and radiological evaluation of simple and complicated Baker's cysts are discussed and the differential diagnosis considered.

Although magnetic resonance (MR) imaging is highly sensitive for the detection of abnormalities of the musculoskeletal system, changes in marrow and soft-tissue signal intensity are often nonspecific, and benign conditions may be mistaken for malignancy. In an effort to identify the cause of this type of misdiagnosis in MR examinations, the MR scans were reviewed of 30 patients whose images manifested bone and/or soft-tissue changes that were initially believed to be consistent with malignancy but were later found to represent benign conditions. Of the 30 patients, MR abnormalities were attributable to trauma in 11, benign tumors and tumorlike conditions in ten, infection in seven, and prior radiotherapy in two. In 24 cases, correlative imaging studies (14 patients), appropriate clinical history (six patients), and/or physical and laboratory examinations (four patients) would have avoided these misdiagnoses.