Genomic insert DNAs from 45 probes representing 113.4 kb of the X chromosome were screened for AC dinucleotide repeat sequence. Two new AC repeat sequences were identified with length polymorphism based on variation in repeat copy number. One at DXS237 exhibits 44% heterozygosity and is potentially useful for rapid diagnosis and mapping of X-linked disorders in Xp22.3. The other, at DXS102 in Xq26, has 71% heterozygosity. This marker will improve accuracy of diagnoses by linkage for families with Borjeson- Forssman-Lehmann syndrome. Review of the literature has identified 31 PCR based markers on the X chromosome, with minimum heterozygosity of 50%, applicable to the mapping and diagnosis of X-linked disorders.