Because of a palpable swelling, 57 popliteal fossae were examined echographically. In 24 cases, the echography revealed a probable Baker's cyst. This diagnosis was confirmed by arthrography or at operation in 21 of the 24 cases. Two of the three findings which did not correspond were due to over-interpretation of the echographic picture and can be avoided in future by giving particular intention to the sound intensity. The causes of the discrepancy in the third case are still undetermined. These results show that echography is suitable for the discovery and assessment of Baker's cysts and should be used first as a non-invasive, painless, rapidly reproducible method.