This paper describes a syndrome of entrapment of the infrapatellar branch of the saphenous nerve attributable to its anatomical relationship to the sartorius muscle and justifies its identification as a distinct entity. Surgery was performed on four knees in three patients for localized pain and tenderness. The infrapatellar nerve was found entrapped behind the sartorius tendon against the prominent edge of the medial femoral condyle and sharply angulated while passing around the sartorius in two knees. Entrapment during passage of the nerve through a slit in the musculotendinous portion of the sartorius was observed in the others. All patients had complete relief of symptoms after translocation of the nerve away from the site of entrapment. The characteristic findings demonstrate the need to be aware of the possibility of entrapment of the infrapatellar branch of the saphenous nerve when evaluating patients with pain on the medial aspect of the knee.