Two cases of the infrequently reported accessory soleus muscle are described. In the first case the muscle was responsible for an intermittently painful swelling behind the medial malleolus and in the second case the muscle was an accidental asymptomatic finding during removal of the plantaris tendon for an ankle ligament reconstruction. The possibility of an accessory soleus muscle should be considered in the differential diagnosis of soft tissue swellings behind the medial malleolus. The CT-scan and EMG can be valuable diagnostic investigations.