One hundred nine runners were treated conservatively without immobilization for overuse injury to the Achilles tendon. Treatment strategies were directed toward rehabilitation of the gastrocnemius/soleus muscle-tendon unit, control of inflammation and pain, and control of biomechanical parameters. One fair, 12 good, and 73 excellent results were reported, with a mean recovery time of 5 weeks. Followup was incomplete in 23 cases. The three most prevalent etiological factors were overtraining (82 cases), functional overpronation (61 cases), and gastrocnemius/soleus insufficiency (41 cases). The authors speculate that runners are susceptible to Achilles tendinitis with peritendinitis due to microtrauma produced by the eccentric loading of fatigued muscle, excess pronation producing whipping action of the Achilles tendon, and/or vascular blanching of the Achilles tendon produced by conflicting internal and external rotatory forces imparted to the tibia by simultaneous pronation and knee extension. Virtually all cases of Achilles tendon injury appear to result from structural or dynamic disturbances in normal lower leg mechanics and require active treatment regimens which attempt to establish normal function to prevent recurrence.