Mitochondrial trifunctional protein (MTP) is a hetero-octamer of four alpha and four beta subunits that catalyzes the final three steps of mitochondrial long chain fatty acid beta-oxidation. Human MTP deficiency causes Reye-like syndrome, cardiomyopathy, or sudden unexpected death. We used gene targeting to generate an MTP alpha subunit null allele and to produce mice that lack MTP alpha and beta subunits. The Mtpa(-/-) fetuses accumulate long chain fatty acid metabolites and have low birth weight compared with the Mtpa(+/-) and Mtpa(+/+) littermates. Mtpa(-/-) mice suffer neonatal hypoglycemia and sudden death 6-36 hours after birth. Analysis of the histopathological changes in the Mtpa(-/-) pups revealed rapid development of hepatic steatosis after birth and, later, significant necrosis and acute degeneration of the cardiac and diaphragmatic myocytes. This mouse model documents that intact mitochondrial long chain fatty acid oxidation is essential for fetal development and for survival after birth. Deficiency of MTP causes fetal growth retardation, neonatal hypoglycemia, and sudden death.