The causes of autism are heterogeneous and predominantly genetically determined. An exact aetiology is found in less than 10% of affected patients. The disappointment about low rates of success in identifying a definite pathology, numerous reports about the association of autism and "metabolic derangements", and rumours of "miraculous cures" after application of various drugs and dietary regimes have resulted in substantial confusion about meaningful diagnostic procedures and rational therapies for subjects with autism. The aim of this report is to give an overview about rare, genetically determined neurometabolic disorders (inborn errors of metabolism) that are evidently (e.g. Smith-Lemli-Opitz Syndrome) or allegedly (e.g. succinate semialdehyde dehydrogenase deficiency) associated with autism-specific symptoms. Affected patients usually display additional neurological symptoms. Procedures required to establish the diagnosis and eventual therapeutic consequences derived from a specific metabolic defect are presented. In addition to these well-defined neurometabolic disorders for which there are rational therapeutic strategies, hypotheses about the association of autism with "metabolic derangements" that could not be confirmed or were clearly falsified are discussed.