To evaluate the ultrasonographic (US) findings in patients with a referring diagnosis of tennis leg and to explore the relative importance of the plantaris tendon and gastrocnemius muscle in the pathogenesis of this condition. A cadaveric study was performed to outline the normal anatomy of the posterosuperficial compartment of the calf. Magnetic resonance (MR) imaging and US were performed, followed by gross anatomic correlation. US findings in 141 patients referred with a clinical diagnosis of tennis leg were retrospectively reviewed by means of consensus of two radiologists. Images were analyzed with respect to the integrity of the lower-leg musculotendinous units, presence of fluid collection, and deep venous thrombosis. MR imaging and US enabled distinction of the musculotendinous unit of the plantaris from the remaining muscles of the lower extremity in cadaveric specimens. US findings in the 141 patients included rupture of the medial head of the gastrocnemius muscle in 94 patients (66.7%), fluid collection between the aponeuroses of the medial gastrocnemius and soleus muscles without muscle rupture in 30 patients (21.3%), rupture of the plantaris tendon in two patients (1.4%), and partial rupture of the soleus muscle in one patient (0.7%). Deep venous thrombosis was seen in isolation in 14 patients (9.9%). In patients with clinical findings of tennis leg who undergo US, abnormalities of the medial gastrocnemius muscle appear to be more common than those of the plantaris tendon.