Medium chain acyl coenzyme A dehydrogenase (MCAD) deficiency presents with episodic fasting, hypoketotic hypoglycaemia, and coma. It is known to be potentially lethal, but the outlook for survivors is thought to be good. We reassessed all patients with MCAD deficiency diagnosed in New South Wales (population six million) to explore long term morbidity and mortality. There were 16 probands and two siblings were confirmed and two presumed to be affected. Assuming an incidence of 1:20,000 births, these represented about 22% of the total number of expected cases. Five (25%) of the 20 patients died aged 3 days-30 months, all during the first episode of illness. Seven others had only one episode and one affected sibling was asymptomatic. Eight had had significant neonatal symptoms. Only two had a significant, serious life threatening episode after diagnosis. Of 15 survivors, one has severe handicap after a single severe episode, and four, aged 9-17 years, have mild intellectual handicap. Eight (including six aged less than 7 years), have apparently normal development. Two are lost to follow up. Our study of unselected patients with MCAD deficiency from a defined population shows not only a substantial risk of death, but also of long term morbidity.