Gas chromatographic retention indices (methylene units) are reported for 101 urinary organic acids as their trimethylsilyl and oximated trimethylsilyl derivatives on a 5% phenylmethyl silicone fused silica capillary column. Using anion exchange chromatography, organic acids were extracted from urines of five healthy individuals, seven patients with neuroblastoma, and nine patients with inherited organic acidurias. Separation of the various acids was achieved by capillary gas chromatography and identification was done by mass spectrometry using a computerized library search program. All identifications were confirmed by visual comparison with reference mass spectra. Standard deviations of the retention indices for all acids were less than 0.035 methylene units and for 46 acids less than 0.01 methylene units. Three chromatograms of urine from individuals with neuroblastoma, phenylketonuria, and propionic acidemia and one from a healthy individual are shown.