The acylcarnitines in plasma and blood spots of 23 patients with proven deficiency of long-chain 3-hydroxyacylcoenzyme A dehydrogenase were reviewed. Long-chain 3-hydroxyacylcarnitines of C14:1, C14, C16 and C18:1 chain length, and long-chain acylcarnitines of C12, C14:1, C14, C16, C18:2 and C18:1 chain length were elevated. Acetylcarnitine was decreased. In plasma, elevation of hydroxy-C18:1 acylcarnitine over the 95th centile of controls, in combination with an elevation of two of the three acylcarnitines C14, C14:1 and hydroxy-C16, identified over 85% of patients with high specificity (less than 0.1% false positive rate). High endogenous levels of long-chain acylcarnitines in normal erythrocytes reduced the diagnostic specificity in blood spots compared with plasma samples. The results were also diagnostic in asymptomatic patients, and were not influenced by genotype. Treatment with diet low in fat and high in medium-chain triglyceride decreased all disease-specific acylcarnitines, often to normal, suggesting that this assay is useful in treatment monitoring.