To assess the maturation process of the cartilage after autologous chondrocyte implantation (ACI), we performed a longitudinal study with three-dimensional spoiled gradient-recalled (3D-SPGR) magnetic resonance imaging (MRI). Five knees of five patients on which ACI of the femoral condyle was performed were studied. The signal intensity of reparative tissue approached that of normal articular cartilage with time. The volume of reparative tissue remained at an almost constant level after implantation. During second-look arthroscopy, the areas on which ACI was performed were covered with hyaline-like cartilage, and the reparative tissue removed by biopsy consisted of normal chondrocytes and extracellular matrix. The increased signal intensity of the reparative tissue represents maturation of implanted autologous chondrocytes. 3D-SPGR MRI is thought to be useful for evaluating reparative tissue after autologous chondrocyte implantation.