['The cause of polycystic ovary syndrome (PCOS) is unknown, although genetic and environmental influences are clearly implicated. Some genetic studies have suggested the involvement of X-linked genes in PCOS, but the influence of X chromosome inactivation (XCI) on manifestation of this disorder has not previously been examined.', 'The objective of the study was to test the null hypothesis that XCI has no influence on clinical presentation of PCOS.', 'We examined patterns of XCI between sister pairs with the same genotype at a polymorphic locus on the X chromosome in families with PCOS.', 'The study was conducted at a private practice.', 'PCOS was defined as hyperandrogenemia with chronic anovulation. Forty families were studied in which DNA was obtained from at least one parent, the proband, and one sister that could be accurately diagnosed as being affected or unaffected.', 'Relative expression of two X-linked alleles was determined, and the ratio of one to the other represented the pattern of XCI.', 'The statistical odds on a different clinical presentation between sisters was approximately 29 times higher in sister pairs with different patterns of XCI, compared with sister pairs with the same pattern of XCI (odds ratio 28.9; 95% confidence interval 4.0-206; P = 0.0008).'] Does epigenetic modification of the X chromosome influence susceptibility to polycystic ovary syndrome?