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<0.001) but not between the peak ammonium level (351 to 1800 μM) and IQ. There was also a significant correlation between CT abnormalities and duration of hyperammonemic coma (r = 0.85, P<0.01) and between CT abnormalities and concurrent IQ (r = −0.75, P<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. (N Engl J Med 1984; 310:1500–5.)