3-Methylglutaconic-aciduria type I (MGA1, OMIM 250950) resulting from 3-Methylglutaconyl-coenzyme A hydratase deficiency is a rare inherited metabolic disorder of l-leucine catabolism. We diagnosed this condition in a 4-year-old German male with generalized fever-associated seizures from the age of 12 months and normal psychomotor development. First he was considered to suffer from uncomplicated febrile seizures. After his eighth seizure, laboratory investigations were performed to exclude inborn errors of metabolism. Analysis of organic acids in urine indicated highly elevated concentrations of 3-methylglutaconic and 3-hydroxyisovaleric acids. 3-Methylglutaconyl-coenzyme A hydratase activity was markedly decreased in skin fibroblasts. Mutation analysis in the AUH gene revealed homozygosity for a novel splice site mutation IVS9-2A>G. We conclude that MGA1 may be associated with fever-associated seizures even in children without delayed psychomotor development.