A rapid and selective capillary zone electrophoretic method for screening of patients with orotic aciduria is described. The method is based on direct measurement of orotic acid in untreated urine. Total analysis time is 5 min with the limit of quantification of 10 mumol/l. Selectivity of the method is given by the use of a low pH of background electrolyte at which practically no interferences from native urine appeared, as well as by using a fast scanning detector which enables the identification of orotic acid via its characteristic UV spectra. This method can be used for quantitative assessment of orotic acid present in urine at physiological conditions after an ion exchange chromatography-clean up and preconcentration procedure. Urine samples containing orotic acid at pathological concentrations (typically more than 1 mol/mol creatinine) could be successfully analysed. The method described was applied to urine specimens collected from both healthy volunteers and patients with orotic acidurias of various origin.