The role of inherited metabolic defects in SIDS is controversial: some workers think that they may account for the cause of death in about 10% of cases. Many maintain that this is a gross overestimate, but it cannot be denied that the sudden onset and rapid deterioration known to occur in some metabolic disorders during the first year of life can mimic SIDS. This may remain undetected unless postmortem material is examined in specialist centres with individual metabolic disorders in mind. Defects in energy metabolism and the maintenance of glucose homeostasis frequently show this pattern of presentation precipitated by minor clinical infection. These disorders include the glycogen storage disorders, gluconeogenic enzyme defects, and the defects of fatty acid oxidation. Several reports have appeared since 1984 linking fatty acid oxidation defects with SIDS. Estimates of their prevalence vary, due partly to methodological heterogeneity and partly to the limited size of individual studies which have not permitted adequate statistical analysis. Nevertheless, the lack of adequate information has not precluded a great deal of debate. Over the past seven years a group in Sheffield has studied the occurrence of these defects in SIDS using three distinct approaches: retrospective analysis of SIDS cases, prospective analysis of SIDS cases, prospective analysis of urine obtained from siblings of SIDS cases during the first week of life. The result of these studies would suggest that the contribution to SIDS made by medium chain acyl CoA dehydrogenase deficiency may be in the region of 1%. Additional related metabolic disorders may account for a further as yet undefined small percentage. The eventual resolution of some of these uncertainties may be provided by harmonizing the results of existing studies and by DNA analysis of materials obtained at necropsy in SIDS cases.