Propionic acidemia is an autosomal recessive disorder caused by deficiency of propionyl CoA carboxylase. Affected patients can develop severe hyperammonemia, whose causative mechanism is unknown. In this study, we monitored changes in metabolic parameters associated with hyperammonemia in patients with propionic acidemia. Levels of ammonia were correlated with plasma levels of individual amino acids and carnitine and with urinary organic acids. Significance of correlations was determined with analysis of variance. Hyperammonemia positively correlated with an increase in branched-chain amino acids (leucine and isoleucine) and a decrease in glutamine/glutamate and esterified carnitine. The urinary excretion of methylcitric acid, formed by the combination of propionic acid with oxaloacetate from the Krebs cycle, increased while that of citric acid decreased with hyperammonemia. These results suggest that in propionic acidemia, hyperammonemia is triggered by catabolism with the accumulation of propionic acid derivatives. The decrease of the plasma levels of glutamine/glutamate with hyperammonemia in patients with propionic acidemia indicates that the mechanism producing hyperammonemia differs from that in urea cycle defects. The increase in methylcitric acid and decline in citric acid urinary excretion suggest that hyperammonemia in propionic acidemia might be related to inability to maintain adequate levels of glutamine precursors through a dysfunctional Krebs cycle.