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.