Background: Acute surgical repair of proximal hamstring ruptures has been shown to result in a high return to preinjury activity level. Hypothesis: Achilles allograft reconstruction of chronic complete proximal hamstring ruptures results in improved function and strength that approaches that of acute surgical repairs. Study Design: Cohort study; Level of evidence, 2. Methods: Between 2002 and 2005, 26 patients underwent surgical treatment for complete proximal hamstring ruptures. Twenty-one were acute primary repairs. Five chronic ruptures were treated with Achilles allograft reconstruction with (n = 2) or without (n = 2) interference screw fixation or mobilization and primary repair (n = 1). Isokinetic strength testing was conducted postoperatively in 11 acute and 3 chronic cases, and a functional questionnaire was given at most recent follow-up. Results: Subjective results for all patients at a mean follow-up of 20 months revealed good leg control in 96%, no pain in 80%, and return to sporting activities in 76%. Ninety-six percent of patients said they would have the procedure done again. With the isokinetic testing available at most recent follow-up, there was no significant difference in the mean hamstring strength deficits for the acute versus chronic groups tested at 60 deg/s (8% vs 21 %, P =.295) and 180 deg/s (12% vs 2%, P =.294). Overall, there were 3 major adverse events and 5 patients with superficial wound drainage treated with antibiotics. Conclusion: Surgical treatment of acute and chronic complete proximal. hamstring ruptures can result in a high return to full activity. Reconstruction of chronic ruptures with Achilles allograft appears to restore function and strength comparable to acute repairs.