We measured the ultrasound scans of 23 (21 males, two females) physically active subjects (mean age: 40 years; range: 20 - 58) with unilateral Achilles tendinopathy to determine the pennation angle in the soleus muscles of the affected and non-affected limb. All scans were performed by one radiologist. The pennation angle was lower on the affected side in 15 patients, and higher on the non-affected side in eight patients. There was a significant difference in the angle of pennation of the soleus muscle on the affected side (15.9 degrees ) compared with that on the non-affected one (17.3 degrees ; p = 0.01). Patients with symptomatic Achilles tendinopathy are more likely to have decreased pennation angle of the soleus muscle on the affected side. This may have implications for appropriate management of Achilles tendinopathy.