A 5-month-old boy with no history of vomiting, early sexual development, or noticeable significant illness was found dead in bed. Autopsy demonstrated bilateral adrenal hyperplasia unequivocally shown on biochemical testing of blood and urine to be due to 21-hydroxylase deficiency. Genetic analysis of the CYP21 gene showed compound heterozygosity; 1 allele contained a pseudogene sequence (gene conversion) and the other contained a previously described I172N point mutation. On theoretical grounds, the genotype would have been expected to cause simple virilizing congenital adrenal hyperplasia but, because no other cause of death could be found, it is possible that it caused a fatally severe loss of enzyme activity in this child. If this assumption is valid, newborn screening would have prevented this death, had it been available.