A 14-year-old girl suffering from severe bilateral late-onset Blount's disease was treated by triple proximal tibial osteotomy. Surgical procedures included dome, respectively, closing wedge valgus osteotomy of the tibia, elevating osteotomy of the medial tibial plateau and lateralising osteotomy of the anterior tibial tuberosity. Mechanical leg axis was restored close to normal, the depression of the medial tibial plateau corrected and the extensor apparatus of the knee realigned. At 7.5 (right side) and 5.5 (left side) months of follow-up, all osteotomies had radiologically consolidated and the patient did not complain of knee pain or instability. Level of evidence IV.