BACKGROUND:A few prospective controlled trials comparing early functional rehabilitation after Achilles tendon repair and non-operative immobilization have been reported. HYPOTHESES:There is no difference in Achilles tendon elongation between early motion and immobilization after Achilles tendon repair. Tendon elongation does not correlate with the clinical outcome. STUDY DESIGN:Randomized clinical trial; Level of evidence, 2. METHODS:Fifty patients with acute Achilles tendon rupture were randomized postoperatively to receive either early movement of the ankle between neutral and plantar flexion in a brace for 6 weeks or immobilization in tension using a below-knee cast with the ankle in a neutral position for 6 weeks. Full weightbearing was allowed after 3 weeks in both groups. Standardized radiographs to measure previously placed radiographic markers were taken on the first day postoperatively and at 1, 3, 6, 12, 24 weeks postoperatively, with the final radiograph a mean of 60 (SD, 6.4) weeks postoperatively. The outcome was assessed at the 3-month and final checkups by the clinical scoring method described by Leppilahti et al and included subjective factors and objective factors. RESULTS:Tendon elongation occurred in both groups but was somewhat less in the early motion group (median 2 mm in the early motion group vs median 5 mm in the cast group a mean of 60 weeks postoperatively, P = .054). The elongation curves first rose and then slowly fell in both groups. The patients who had less elongation achieved a better clinical outcome (rho = -.42, P = .017). Tendon elongation did not correlate significantly with age, body mass index, or isokinetic peak torques. CONCLUSION:Achilles tendon elongation was somewhat less in the early motion group and correlated with the clinical outcome scores. We recommend early functional postoperative treatment after Achilles rupture repair.