One of the metabolites of ochratoxin A (OA) is 4R-4-hydroxyochratoxin A (4-OH-OA), and the ratio of 4-OH-OA to OA excreted in urine can be linked to the carcinogenic potential of this compound. As further support to the hypothesis that OA can be involved in Balkan endemic nephropathy and the associated urinary system tumours, it was decided to investigate the presence of these two compounds in the urine of affected populations. A sensitive method is described for the determination of the compounds at the 10 ng l-1 level. It involves extraction, two purification steps by column chromatography and high-performance liquid chromatography (HPLC), an analysis by HPLC and a confirmatory test by HPLC after derivatisation.