Gas chromatography/mass spectrometry was used to identify a series of acids in urine and serum from a child who died 26 h after birth in severe metabolic acidosis with high lactate excretion. cis-5-Decene-1, 10-dioic acid and cis-5-dodencene-1, 12-dioic acid were synthesized and used as references. The following acids were found: hexane-1,6-dioic acid, octane-1,8-dioic acid, decane-1,10-dioic acid, dodecane-1,12-dioic acid, cis-5-decene-1,10-dioic acid, cis-5-dodecen-1,12-dioic acid, cis-5-tetradecene-1,14-dioic acid, trans-3-decene-1,10-dioic acid, and trans-3-dodecene-1,12-dioic acid. The concentration of C6 to C14 acids in the patient's urine was 3.7 mol/mol of creatinine; it was less than 0.2 mol/mol of creatinine in eight normal newborns and approximately 0.1 mol/mol of creatinine in a case of fructose-1,6-biphosphatase deficiency with lactic acidosis. 5-cis-Dodecenedioic acid was present in highest concentration: 1 mol/mol of creatinine in urine and 61 mumol/liter in serum. We propose that impaired beta-oxidation, probably at the acyl-CoA-dehydrogenase step, resulted in the formation of the observed acids. The parents were consanguineous, and a sibling died with the same clinical picture, which suggests a genetic defect.