Deformity of the lower extremities in 26 patients with multiple cartilaginous exostosis was examined radiologically. Follow-up periods ranged between 3 and 33 years (mean 10.3 years). Twenty-four patients had deformity of the joints. A femoral neck-shaft angle (FNA) of more than 150 degrees was noted in 14 patients (26 of 51 hip joints) at diagnosis. After approximately 10 years of age, the FNA tended to decrease. Eleven patients (22 of 52 knee joints) had genu valgum (the femorotibial angle < mean -2 SD of normal control) which was caused by valgus deformity of the distal femur in one-third of the patients and that of the proximal tibia in two-thirds. Fifteen of 21 patients (29 of 42 joints) had valgus deformity of the ankle (antero-posterior mortise angle of the ankle > 94 degrees), and in half of them, the valgus deformity progressed with growth. Two patients (aged 10 and 11 years) underwent varus osteotomy of the tibia with partial excision of the fibula. However, their deformity relapsed. Surgical treatment for hip deformity is unnecessary during the growth stage. Progressive deformity of the knee and ankle should be detected in an early stage, and the surgical indication has to be examined.