Various types of dicarboxylic aciduria are known, most of them are accompanied by non-ketotic hypoglycaemia. For the differential diagnosis of these conditions several methods of investigation have been used: (1) analysis of urinary organic acids in both native and hydrolysed samples, (2) analysis of free and esterified carnitine, the latter by means of chromatographic separation and identification of acyl moieties, (3) analysis of plasma organic acids, including the so-called free fatty acids, (4) a prolonged fasting test with serial measurements of the aforementioned parameters and close monitoring of the blood glucose and (5) an oral loading test with medium chain triglycerides accompanied by the same measurements as those named in item (4). So far differentiation has been made between patients with a metabolite profile most probably characteristic of medium chain acyl-CoA dehydrogenase deficiency and other dicarboxylic acidurias, among the latter systemic carnitine deficiency. Patients belonging to the first group accumulate octanoate, decanoate and cis-4-decenoate in their plasma; they excrete hexanoylglycine, octanoylcarnitine and suberylglycine in addition to the usual C6-C10 dicarboxylic acids. There was a high prevalence of an increased plasma free fatty acid/3-hydroxybutyrate ratio.