We studied 26 children with inborn errors of urea synthesis who survived neonatal hyperammonemic coma. There was a 92 per cent one-year survival rate associated with nitrogen-restriction therapy and stimulation of alternative pathways of waste nitrogen excretion. Seventy-nine per cent of the children had one or more developmental disabilities at 12 to 74 months of age; the mean IQ was 43 +/- 6. There was a significant negative linear correlation between duration of Stage III or IV neonatal hyperammonemic coma and IQ at 12 months (r = -0.72, P less than 0.001) but not between the peak ammonium level (351 to 1800 microM) and IQ. There was also a significant correlation between CT abnormalities and duration of hyperammonemic coma (r = 0.85, P less than 0.01) and between CT abnormalities and concurrent IQ (r = -0.75, P less than 0.02). These results suggest that prolonged neonatal hyperammonemic coma is associated with brain damage and impairment of intellectual function. This outcome may be prevented by early diagnosis and therapy.