We reviewed the clinical and radiographic characteristics and response to treatment of 11 children (16 knees) with idiopathic late-onset tibia vara (adolescent Blount's disease) followed for an average of 5.7 years (range, 3-10 years). The clinical characteristics were very consistent: black race; 2:1 male predominance; normal height; marked obesity; knee pain as the primary presenting complaint; and slowly progressive genu varum deformity that averaged 19 degrees (range, 10-45 degrees). Radiographically, the epiphyses were wedge shaped owing to medial flattening, the physes were irregular in thickness, and there was minimal, if any increased prominence of the proximal medial metaphysis. Nineteen proximal tibial valgus and diaphyseal fibula osteotomies were performed on 15 knees. There was a 50% rate of recurrent deformity in males with clinical onset at less than or equal to 10 years of age. Females and older males had no recurrences. Histopathologic studies of the physis performed on one case demonstrated abnormal cellular islands of hyaline cartilage, small foci of necrotic cartilage, prominent intertrabecular vascularity, and premature medial physeal closure. These data support late-onset tibia vara as a distinct entity closely related to the infantile form. They also suggest three specific forms of tibia vara based on the age at clinical onset: infantile (0-3 years), juvenile (4-10 years), and true adolescent (11 years or older). The juvenile group is characterized by a high rate of recurrence following surgical correction, whereas the others are not.