We reviewed the natural history and differential diagnosis of ornithine transcarbamylase deficiency (an X-linked inborn error of urea synthesis) in 13 symptomatic female heterozygotes. The patients presented as early as the first week of life or as late as the sixth year. The most common symptoms before diagnosis were nonspecific: episodic extreme irritability (100 percent), episodic vomiting and lethargy (100 percent), protein avoidance (92 percent), ataxia (77 percent), Stage II coma (46 percent), delayed physical growth (38 percent), developmental delay (38 percent), and seizures (23 percent). Including the proband, 42 percent of the female members of the 13 families studied had symptoms. The median interval between the onset of major symptoms (vomiting and lethargy, seizures, and coma) and diagnosis was 16 months (range, 1 to 142). Five patients had IQ scores below 70 at the time of diagnosis. We suggest that careful evaluation of the family history, the dietary history, the episodic nature of the nonspecific symptoms, the response of these symptoms to the withdrawal of protein, and their frequent onset at the time of weaning from breast milk will permit early diagnosis and might thereby reduce the risk of death or neurologic impairment in female patients with partial ornithine transcarbamylase deficiency.