Odd-chain fatty acids are recognized precursors of propionate in man, but their clinical significance in disorders of propionate metabolism has not been well studied. Urinary excretion of methylmalonate, methylcitrate, propionylglycine, and 3-hydroxypropionate was measured in five children with methylmalonic acidemia and three with propionic acidemia during frequent or continuous feeding and after 10-18 h of fasting. There was a significant (p less than 0.01) increase in the mean total measured metabolite excretion during fasting (fed 38.1 mumol/kg/h, fasting 54.6 in methylmalonic acidemia, fed 1.45, fasting 2.98 in propionic acidemia). Percentage rises in each subject were similar for all measured metabolites. These increases in metabolite excretion are most easily explained by mobilization and oxidation of odd-chain fatty acids in the fasting state. Prolonged fasting should be avoided in children with disorders of propionate metabolism.