A 4 1/2-year-old girl with a so far unremarkable medical history became comatose during a simple infection. She showed severe metabolic acidosis without elevation of lactate. In blood the branched-chain amino acids were increased. In urine ketone-bodies, increased 3-OH-isovaleric and 3-OH propionic acid excretion were detected, while methylmalonate was not found. The profile of acylcarnitines revealed increased propionylcarnitine. Despite restriction of protein supply, high-caloric nutrition, correction of acidosis, and supplementation of biotin and carnitine, the girl died 2 days after admission due to arrhythmia of the heart. In skin fibroblasts the activity of propionyl-coenzyme A carboxylase (PCC) was markedly decreased. Mutation analysis confirmed the diagnosis of propionic acidemia (PA) with compound heterozygosity for 2 new missense mutations L417W/Q293E in the PCCA gene, with the mother carrying the Q293E and the father the L417W mutation. Late-onset PA should be included in the differential diagnosis of unclear coma. Determination of the acylcarnitines using tandem mass spectrometry as well as organic acids in urine is recommended.