We report on monozygotic twins with different clinical phenotypes of X-linked adrenoleukodystrophy. At the age of 10 years both boys were neurologically asymptomatic. The first cranial magnetic resonance examination showed normal findings in the first twin and parietooccipital demyelination in the second. The latter developed behavioral problems 9 months later, followed by visual impairment and gait ataxia. His cranial magnetic resonance image at the age of 11 years showed progressive demyelination. In contrast, neurological status and magnetic resonance images remained normal in the first twin. The same point mutation in exon 8 of the adrenoleukodystrophy gene (C2203T) was detected in both boys. All genotype examinations were consistent with the diagnosis of monozygotic twins, suggesting that some nongenetic factors may be important for different adrenoleukodystrophy phenotypes.