Popliteal fossa and distal thigh swellings have many causes in young active adult. Cysts are the main cause, but they may also evolve from fatty tissue, lymph nodes or the popliteal artery. Semimembranosus muscle hypertrophy, found in athletes and young active adults, is only rarely the main or only cause of swelling, and only four such cases are reported in the literature. The present article reports our experience in the management of a case of semimembranosus hypertrophy as the only cause of a swelling above the popliteal fossa, and its favorable evolution.