The purpose of the current study was to evaluate the remodeling process of Achilles allograft for anterior cruciate ligament reconstruction under light microscopic and electron microscopic evaluations. In 12 patients, histologic examinations were done preoperatively and during arthroscopy at 6, 12, and 24 months. Gross observations were made with respect to width, vascularity, elasticity, and stability of the Achilles allograft. Biopsy specimens were obtained at the followup arthroscopic study. Light microscopic examinations done at 6 months showed many fibroblasts with spindle-shaped nuclei and hypercellularity. At 12 months, the small folds pattern of collagen bundles and decreased cellularity were observed. At 24 months, gross and findings of the light microscopic evaluations of the allograft tendons were similar to those of the normal anterior cruciate ligaments. On electron microscopic examinations done at 24 months, the allograft had fibroblasts with much cytoplasm and densely packed parallel-laid collagen fibrils, which showed the characteristic cross striations, but the Achilles allograft ligaments did not show similar findings compared with biopsy samples from normal anterior cruciate ligaments.