Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is an autosomal recessive disorder that has been associated with sudden infant death syndrome and a condition resembling Reye's syndrome. The point mutation K329E is thought to be the commonest mutation causing MCAD deficiency in caucasian patients. The prevalence of this mutation was determined by use of dried blood spots on Guthrie cards obtained during neonatal screening programmes. 12 carriers were identified among 479 newborn babies in Britain, 5 among 353 in Australia, and 5 among 536 in North America but none among 500 in Japan. Since presymptomatic diagnosis and appropriate dietary management can prevent life-threatening episodes in MCAD deficiency, population-based DNA screening for this potentially fatal disorder might be justified in countries with a high frequency of this mutation.