A histological and electron microscopic study were conducted in order to observe a morphological changing pattern of patellar ligament autograft after endoscopic ACL reconstruction. Seventeen patients among 117 consecutive series which underwent endoscopic ACL reconstruction from January 1990 to March 1995, were involved in this study according to the various follow-up periods. In second-look arthroscopic finding at 6 months follow-up after ACL reconstruction, reconstructed ACL was covered with synovium and round-shaped, and after the biopsy active bleeding could be seen. In histological study, 6 months follow-up, spindle or ovoid shaped fibroblast with hypercellularity, high crmp patterns of collagen fiber, and parallel arrangement of fibroblast to collagen fiber were noted. The cellularity of fibroblast decreased in according to periods of follow up. At 12 months follow up, spindle or ovoid fibroblast, its cellularity and arrangement pattern and crimp pattern of collagen fiber seemed to be almost similar to those of normal ACL. This similaryity continued up to 2 years and 2 months after reconstruction. In EM study, until 6 months follow-up after reconstruction, the graft showed randomly arranged fibroblasts with hypercellularity, which had abundant cytoplasm with marked irregular cytoplasmic borders and prominent nuclear indentation, Collagen fibrils were immature and showed a unimodel distribution with small diameter. At 12 months follow-up, elongated fibroblasts appeared, but these cells contained folded nuclei and abundant cytoplasm. Collagen fibrils had parallel arrangement and sparse cross striation, and comprised of mainly small fibrils and scattered large fibrils. After 24 months follow-up, the cellularity decreased more. The regular arrangement of collagen fibrils with wavy pattern similar to normal anterior cruciate ligament was noted. Two distinctive population of the small and large diameter of fibrils was also observed (bimodal distribution). At 26 months, the large and small collagen fibrils compresed a characteric bimodal pattern with parallel arrangement, but the majority of fibroblasts had irregular cytoplasmic contour, and abundant cytoplasm with increased organelles, which indicated the active function. These findings suggest that the graft are still immature even at 26 months in postoperative follow up despite considerable maturation of collagen fibril.