We studied X inactivation patterns in manifesting carriers of familial and sporadic Duchenne muscular dystrophy (DMD) or unaffected carriers of DMD by analysis of the methylation of HpaII sites in the first exon of the human androgen-receptor gene (HUMARA) from peripheral blood samples. Three of the four manifesting carriers, four of the five asymptomatic carriers, and 31 of the 32 female controls were heterozygous for the CAG repeat of HUMARA. All manifesting carriers showed skewed X inactivation, while all unaffected carriers showed almost symmetrical inactivation. One family studied over three generations is noteworthy because it includes two mother/daughter pairs, one an affected pair with skewed X inactivation, and the other a phenotypically normal carrier pair with random X inactivation. On the other hand, the extent of X inactivation for each X chromosome in 31 female controls was widely distributed. These data suggest that in carriers of DMD, both affected and unaffected, it is valuable to analyze the pattern of skewed X inactivation because it provides important prognostic information. Carriers of DMD with skewed X inactivation might show slowly progressive myopathy with advancing age.