Ornithine transcarbamylase (OTC) deficiency, an X-linked inborn error of the urea cycle, leads to the accumulation of ammonia, causing neurologic deficits. Clinical management for the patients with OTC deficiency is frustrating and requires a burdensome medical regimen, since they may have impairment and recurrent episodes of hyperammonemia in spite of intensive care. Therefore, prenatal diagnosis of the affected fetus is important in genetic counselling for the family at high risk. In this study, mutations in the OTC gene of three obligate heterozygotes and a proband have been identified in four unrelated families: R141Q, R320X, H214Y, M205T. Each mutation altered restriction recognition sites; TaqI for R141Q, NlaIII for M205T, RsaI for H214Y, BclI for R320X. Based on their molecular defects, prenatal diagnoses of 6 fetuses including one set of fraternal twins were successfully made at the ninth to eleventh week of gestation by polymerase chain reaction (PCR)-restriction digestion using genomic DNA from chorionic villus sampling (CVS). We predicted the outcome of all fetuses prenatally. Among six, four were females and two were males, which were determined by PCR amplification of the sex determining region of the Y chromosome (SRY) gene. Each carried a wild type allele for the corresponding mutant allele. They were also tested postnatally for the mutations to be unaffected.