Skewing of X inactivation may contribute to the manifestation of symptoms in adrenoleukodystrophy carriers. We observed highly skewed X inactivation in 32% of 22 symptomatic and asymptomatic carriers but not in 7 related and 35 unrelated controls. Skewing of X inactivation correlated with clinical neurological scores but not with the extent of very long-chain fatty acid accumulation. Transcript analysis in cultured fibroblasts revealed that skewing could occur both in favor of the mutant and the wild-type allele. Adrenoleukodystrophy carriers are more susceptible to develop skewing of X inactivation in favor of the mutant allele being associated with the manifestation of symptoms.