Histopathologic and histochemical studies were performed on the entire physes and contiguous structures of five knees (three patients) with late-onset tibia vara (Blount's disease). A constellation of abnormalities of both medial and lateral physeal components resulted in disorganization and misalignment of the physeal zones. The changes were remarkably similar to those observed in both infantile tibia vara and slipped capital femoral epiphysis, suggesting a common etiology. Our evaluation indicates that asymmetric compressive and shear forces acting across the proximal tibial physis effect its disruption and causes suppression and deviation of normal endochondral ossification, thereby resulting in tibia vara.