Lesch-Nyhan disease (LND) is an inborn error of purine metabolism caused by defective activity of the enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT, EC 2.4.2.8), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classic phenotype occurs almost exclusively in males and is characterized by hyperuricemia, mental retardation, severe dystonia, and self-injurious behavior. Heterozygous carrier females are usually clinically normal. However, a small number of clinically affected females have been described. In all previous cases there was a mutation in one HPRT allele and non-random inactivation of the X chromosome carrying the normal HPRT gene. We have analyzed a female MZ twin pair discordant for Lesch-Nyhan disease. The mother and both twins are heterozygous carriers of a HPRT splicing mutation (IVS8 + 4A > G; c.609 + 4A > G) and all three express the mutant allele at similar frequencies in peripheral blood T cells. The mother and one sister are clinically normal. In the affected twin, the clinical phenotype is classical for Lesch-Nyhan disease, despite the fact that HPRT activity in the blood was also normal. X inactivation analysis showed a skewed pattern in the fibroblasts of the affected twin sister, with the X chromosome carrying the normal HPRT allele preferentially inactivated. As in many other reported cases of X-linked diseases, the discordant phenotype of the two monozygous twin sisters suggests that the process responsible for monozygotic twinning can trigger skewed X inactivation.