Male patients with ornithine transcarbamylase (OTC) deficiency usually develop a hyperammonemic crisis in the neonatal period or in early infancy. In contrast to these classical patients, here we report the occurrence of males with the disease who developed a hyperammonemic crisis in adolescence or in late adulthood. The molecular basis for the lateness of the onset of this disease in these patients was investigated. Of 5 cases, cases 1 through 4 each developed a first episode between 16 and 58 years of age, had an OTC activity in the postmortem liver tissue that ranged from 2-9% of control mean, and each had decreased levels of the OTC protein. Analysis of the OTC gene disclosed an R40H mutation in cases 1, 3, and 4, and in a grandson (case 5) of a male sibling of case 1, and a Y55D mutation in case 2. The activity of the mutant OTCs expressed in cos 1 cells was 20% for the R40H mutation, and 30% for the Y55D mutation of the wild type OTC. When the mutant enzyme was treated by freezing and thawing, the R40H OTC decreased to 10% of the mean of the wild type, while the Y55D OTC remained essentially at the 30% level. These results suggested that these mutant enzymes were operating in vivo at a level consistent with low normal urea production, but this level declined due to their structural instability, after an environmental load was applied.