I describe a chromatographic procedure for quantification of urinary estriol in late pregnancy, with 6 alpha-hydroxyestriol used as internal standard. The hydrolyzed sample is successively passed through a Bond Elut C18 column and a DEAE-Sephadex column. The estrogens are then completely resolved as silyl ethers on a packed gas-liquid chromatographic (GLC) column in isothermal mode. Analytical recoveries of conjugated estriol ranged between 96 and 102%. Between-day precision (CV) was 4.4% at a mean concentration of 85.1 mumol/L. Specificity was assessed by comparing results obtained with a capillary GLC column.