Cardiomyopathy and leukodystrophy are life-threatening complications of multiple acyl-CoA dehydrogenase deficiency (MADD). A 2-year-old boy with this disorder developed rapidly progressive leukodystrophy resulting in complete paralysis within 4 months. Within a week of starting sodium-D,L-3-hydroxybutyrate he had improved. After 2 years, neurological function returned, including walking independently, with progressive improvement of brain MRI. Two additional infants with MADD developed life-threatening cardiomyopathy unresponsive to conventional treatment. On sodium-D,L-3-hydroxybutyrate treatment their cardiac contractility showed progressive and sustained improvement. D,L-3-hydroxybutyrate is a therapeutic option for cerebral and cardiac complications in severe fatty acid oxidation defects.