Cytostatic anticancer drugs are an increasingly important issue in the environmental debate, mainly due to the lack of knowledge about the fate of these toxic substances. Over the last decades, 5-fluorouracil (5-FU) has been one of the most frequently used antineoplastic agents and may, therefore, be regarded as one of the pilot substances for environmental contamination. As a prerequisite for these investigations, a method for the determination of 5-FU in hospital effluents has been developed. Waste water samples were enriched by solid-phase extraction on ENV(+) columns (concentration factor 500) and analysed by capillary electrophoresis using a buffer containing 80% 160 mM sodium borate buffer (pH 9.5) and 20% acetonitrile (v/v). This method is applicable within the range 5-500 microg 5-FU L(-1). The standard curve (correlation coefficient >0.99) was linear with recovery rates from 80 to 96% and an accuracy from 9.0 to 20% (intra-assay standard deviation 0.7-8.9%; inter-assay standard deviation 2.2-9.5%). Based on the consumption of 5-FU in oncologic departments and the detection limit of 1.7 microg L(-1), this method covers the range necessary to evaluate 5-FU in hospital effluents. The applicability of the method was proven by chemical analysis of real hospital waste water samples. Obtained over a time period of 1 month (range 20-122 microg 5-FU L(-1)), the results were similar to those calculated by an input-output model. The presented method provides an analytical tool necessary to face the monitoring of environmental contamination problems.