When an accessory soleus muscle is present, it consists of a soft-tissue mass bulging medially between the distal part of the tibia and the Achilles tendon. It usually inserts with a separate tendon on the calcaneus anteromedial to the Achilles insertion, and may be a cause of pain on exercise. One may suspect a soft-tissue tumor, such as lipoma, hemangioma, and even sarcoma, but the anomalous muscle has a typical appearance on plain radiographs, and the appearance on computed tomography is diagnostic. If the patient is asymptomatic, no therapy is required, but if pain or other discomfort is provoked by exercise, exploration with fasciotomy or excision of the accessory muscle is recommended, as was done in six of our eleven patients who were seen between 1968 and 1985.