1. 1. The patient, a boy, was suffering from episodes of vomiting, dystrophy, muscular hypotonia and weakness, psychomotoric retardation, respiratory tract infections, and attacks of metabolic acidosis from the first days of life until death at the age of 10 months. 2. 2. A small amount of serum (1.5 ml), obtained by heart puncture shortly after death, was the only material left from the patient. The serum contained 3.5 mmol/l of propionic acid, 1.13 mM of methylmalonic acid, and large amounts of β-hydroxybutyrate and lactate. In addition, the analyses revealed 1.54 mM of β-hydroxy- n-valeric acid, a compound not detected in human material before. The fatty acid pattern showed increased amounts of the odd-numbered pentadecanoic acid and heptadecanoic acid. There were elevated amounts of lysine, leucine, isoleucine and valine, while the other amino acids were within the normal range. 3. 3. The clinical picture as well as the biochemical findings leave no doubt that the patient suffered from a metabolic disorder, most likely an inborn error of metabolism. The primary defect of the patient was most likely located to the conversion of methylmalonyl CoA to succinyl CoA. Since accumulation of propionate has not been described in patients with a defective methylmalonyl-CoA mutase, the patient may possibly have a defect located to another enzyme in this metabolic sequence, probably the methylmalonyl CoA racemase. The β-hydroxy- n-valeric acid and the odd-numbered long-chain fatty acids are likely to stem from the accumulated propionyl CoA, which may compete with acetyl-CoA in several condensing reactions.