64 patients with insertional Achilles tendinopathy were randomized to receive extracorporeal shockwave therapy (ESWT) and an arginine supplement or ESWT and a placebo. Although there was no difference between the two groups for pain at 2 months, the supplement group demonstrated lower pain scores than the control group at 6 months. The supplement group was also superior to the control group at 2 and 6 months for mean Ankle-Hindfoot Scale scores and Roles and Maudsley scores. At 6 months, the decrease in tissue oximetry values was significantly greater in the supplement group than in the control group.