1. The usual popliteal cyst is a distended semimembranosus bursa, frequently communicating with the joint through a posterior defect which is rather constant in location.
2. Successful excision of the cyst without recurrence does not ensure a successful result in all patients. The cyst may be secondary to an obscure or concealed joint lesions.
3. A lesion of the posterior portions of the medial meniscus should be suspected, even though it is not evident in the roentgenograms, when the cyst has, or has had, an associated joint effusion of unknowns etiology.
4. Early diagnosis and surgical treatment of the meniscal lesions will prevent the formations of such cysts.