3-Hydroxyacyl-CoA dehydrogenase (HADH) deficiency is a rare disorder of mitochondrial fatty acid beta- oxidation. Affected individuals typically present during the first year of life with hypoglycemia, seizures/convulsions and hyperinsulinism. A single patient has been described who did not have hyperinsulinism however this patient had a very high residual enzyme activity (35% of controls in isolated mitochondria). A Reye- like presentation and fulminant hepatic failure has also been reported. Severe developmental disabilities were observed in patients in an affected sibship when treatment was delayed. Most patients have been responsive to diazoxide, which has been reported to be effective in treating severe recurrent hypoglycemia.