We examined seven ankles with an accessory soleus muscle to determine if the magnetic resonance (MR) imaging features were sufficient to allow accurate diagnosis of this anomalous muscle. We believe that the diagnosis of an accessory soleus muscle is unequivocal with MR imaging on the basis of morphology and signal intensity. Therefore, we suggest that this imaging method should be utilized in patients suspected of having a mass or with persistent swelling in the region of the ankle with or without pain, for the noninvasive diagnosis of this muscle and to better define the regional anatomy presperatively if surgery is being considered for relief of symptoms.