Urinary organic acid profiles in patients with inherited defects of fatty acid metabolism and ketogenesis are described. Medium-chain acyl-CoA dehydrogenase, short-chain acyl-CoA dehydrogenase, multiple acyl-CoA dehydrogenase, and 3-hydroxy-3-methyl-glutaryl-CoA lyase deficiencies can be recognized at the metabolite level. Data on long-chain acyl-CoA dehydrogenase and systemic carnitine deficiencies are scarce. In the latter disorders, dicarboxylic aciduria is rather nonspecific and points to a modest omega-oxidation of long chain fatty acids.