Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder of urea synthesis. Among females who carry a mutant OTC allele, there is a wide range of phenotypic variability, ranging from apparent normality to a severe onset and the resulting profound neurologic impairment observed in hemizygous males. This study was designed to define the phenotypic variability of OTC deficiency in ostensibly healthy carrier females and to compare them to noncarrier females from their own and other families. One hundred seventy-five women from 89 families participated in this study. Each completed a mailed questionnaire, allopurinol testing, and fasting plasma amino acid determinations. OTC carrier status was determined by pedigree analysis, allopurinol test results, and/or DNA mutation analysis. Overall, 79 women were identified as carriers of a mutant OTC allele (60 proband mothers, 19 relatives), and 96 women (32 proband mothers, 64 female relatives) were determined to be noncarriers. Comparison of biochemical phenotypes indicated that carriers and noncarriers do not differ in daily urinary creatinine excretion, but that carriers excrete significantly less urea nitrogen and total nitrogen, reflecting their significantly lower historically reported daily protein intake. Carriers had significantly higher levels of fasting plasma glutamine and alanine, and significantly lower levels of citrulline and arginine compared with noncarriers. Carriers and noncarriers reported similar demographic characteristics, anthropometric measurements, level of education, and medical and pregnancy histories. There was no indication of increased incidence of migraine headaches among carriers. Thus, we found no evidence that asymptomatic adult female OTC heterozygotes are at increased risk for previously unidentified health problems apart from an unknown risk for hyperammonemic encephalopathy as occurred in 3 of the carriers in this study. Because these episodes appear to be related to physiologic stress (fracture, parturition), it would seem medically prudent for carriers to be aware of this risk.