X-inactivation has been studied in a series of monozygotic female twins and their female relatives by a PCR method which detects methylation at the androgen receptor locus (HUMARA). The results obtained are compared to those from an earlier study employing probe M27 beta which detects locus DXS255. Analysis of X-inactivation in girls with Rett syndrome and their mothers by four different methods did not indicate a direct relationship between non-random inactivation of the X-chromosome and the presence of the disease with the exception that any skewing detected in the probands tended to favour the preferential inactivation of the paternally inherited X-chromosome. No evidence for the involvement of uniparental disomy in the etiology of the disease was found.