Eight patients with synovial cysts about the knee were studied by computed tomography (CT). Six of the eight patients had cysts in atypical locations, four medial and two anterior. Knee arthrography, performed on five patients, demonstrated the cyst in only two cases. In all patients CT defined the location and extent of the cyst, demonstrating the following benign features: a well-defined homogeneous mass with attenuation values approximating that of water, preservation of adjacent soft-tissue planes, and absence of bone involvement. The authors conclude that CT is useful in the evaluation of a suspected synovial cyst when its position is atypical or when arthrography fails to demonstrate or completely delineate the cyst. CT provided diagnostic accuracy and determination of the optimal operative approach.