Of 34 consecutive proximal tibial osteotomies for secondary degenerative arthritis in patients under 40 years of age, 33 were evaluated at least three years (mean 7.5 years) after operation. In all 73% were satisfactory, with four failures in 21 procedures in men and five failures in 12 procedures in women. The primary abnormalities were medial meniscectomy (11), medial and lateral meniscectomy (4), osteochondritis dissecans (3), osteochondritis dissecans with medial meniscectomy (4) and fracture (11). All four knees with both medial and lateral meniscectomy had unsatisfactory results despite obtaining anatomical alignment. Eight patients needed subsequent surgery; five of them had total knee replacement, four within four years of the osteotomy. Proximal tibial osteotomy in younger patients with secondary arthritis gives similar results to those for older patients with primary osteoarthritis. If it fails, this is generally within the first four years after the operation.