The absolute configuration of urinary 5-hydroxyhexanoic acid was determined by means of capillary gas-liquid chromatography of the O-D-2-phenylpropionylated methyl ester derivatives on SE-30 as stationary phase. In three patients with non-ketotic dicarboxylic aciduria and one patient on a diet containing excessive amounts of medium-chain triglycerides, - 70% of the L-isomer and - 30% of the D-isomer were found. On the basis of these results it is concluded that 5-hydroxyhexanoic acid is a 'normal' degradation product of fatty acids.