In recent years, epidemiological evidence that exposure to toluene diisocyanate (TDI) is associated with adverse health effects has led to the development of useful analytical methods for the biological monitoring of TDI. In this paper, an HPLC method is presented that allows accurate determinations of toluenediamines (TDA), urinary metabolites of TDI, in hydrolysed human urine without complicated or time-consuming sample treatment. The procedure requires 5.0 ml of urine and involves the extraction with toluene of TDA and the hydrolysable conjugate fraction followed by further purification with a strong cation-exchange sorbent. Strongly alkaline conditions are chosen for the hydrolysis of urine samples and phenylene-1,3-diamine is used as internal standard to control the sample extraction and clean-up. Separation is performed on a base-deactivated octadecyl reversed-phase column by either ion-suppression or ion-pair chromatography. Chromatographic analysis is completed in less than 20 min and chromatograms with no interfering peaks are obtained. High sensitivity and selectivity are achieved by using electrochemical detection: 2,6-and 2,4-TDA can be detected at the 0.1 and 0.15 µg l^–1 levels, respectively. Absolute recoveries of the method tested with urine samples spiked at 10 µg l^–1 with phenylene-1,3-diamine and from 1 to 25 µg l^–1 with 2,6-and 2,4-TDA are greater than 87.6% and 88.3%, respectively. The assay is linear from 0 to 50 µg l^–1. Within-run precisions evaluated on 10 urine samples ranging from 0 to 10 µg l^–1 are 7.9% and 5.3% for 2,6-and 2,4-TDA, respectively. Results obtained with urine samples from 12 controls and 15 exposed workers from a flexible polyurethane foam factory indicate that the method is appropriate for the biological monitoring of occupational exposure to TDI.