Hydatid cysts rarely involve the musculoskeletal system. We present the case of a 23-yearold man with a primary hydatid cyst between the femur and the quadriceps muscle in his left thigh. No cysts were located in the adjacent femur or quadriceps muscle. Cyst resection with sparing of the surrounding muscles, combined with anthelmintic therapy, was curative. In regions where hydatidosis is endemic, hydatid cysts should be included in the differential diagnosis of any unusual soft-tissue swelling. (C) 2004 Wiley Periodicals, Inc.