The authors present the cases of monozygotic male twins with right-sided Legg-Calvé-Perthes disease (LCPD) with different formation of the lumbosacral junction. This is likely the fi rst description of a lumbosacral junction formation disorder associated with identical twins who were both treated for LCPD as children. The disease began at 6 and 9 years of age and during treatment as well as in adulthood signifi cantly different bone formation of the lumbosacral transitional vertebra, was observed in both brothers. Twin A has a unilateral right-sided fusion of the enlarged L5 transverse process with the ipsilateral sacral ala, twin B has a complete sacralization of the fi fth lumbar vertebra. The LCPD treatment outcomes in the twins were consistent with the results from large studies, i.e., age at the time of LCPD onset is the main factor infl uencing the prognosis, however the morphological difference in the transitional vertebrae in these monozygotic twins was signifi cantly. Key words: lumbosacral transitional vertebra, lumbosacral junction formation, sacralization of lumbar vertebra, megatransverse of vertebra L5.