We evaluated the remodeling process of autogenous patellar tendon graft for anterior cruciate ligament (ACL) reconstruction by means of light microscopic (LM) and electron microscopic (EM) examinations from the biopsy specimens obtained at the time of second-look arthroscopy. Twenty-one patients were examined at various times postoperatively (from 6 weeks to 15 months, mean 9.5 months), and the results were correlated with the morphology of normal patellar tendon and normal ACL. Our study showed that the graft was revascularized in the early postoperative period, fibroblastic remodeling took place, and the graft obtained gross similarity to the original ACL on their arthroscopic and LM appearances at approximately 1 year postoperatively. However, EM study showed that at both approximately 6 months and 1 year postoperatively the grafts consisted equally of active fibroblasts with a higher cytoplasm-to-nucleus ratio compared with normal ACL. Collagen fibrils of these grafts were of uniformly small diameter compared with normal patellar tendon and ACL. Our results with ultrastructural study suggest that the grafts were still immature even at 1 year postoperatively.