A simple high-performance liquid chromatographic (HPLC) method for simultaneous determination of proguanil and its active metabolite cycloguanil in human urine has been developed. Quinine sulphate was used as the internal standard. The assay uses a reversed phase C18 microbore column (2 mm I.D. × 10 cm) packed with 3 μm ODS Hypersil. The chromatographic separation was achieved by using an isocratic mobile phase comprising acetonitrile-aqueous phosphate buffer (10:90, v/v) containing 200 mM sodium dodecyl sulphate adjusted to pH 2. The mobile phase was pumped at 0.4 ml/min. The eluant was monitored by a UV detector operating at 254 nm. The assay was based on an organic extraction with 1-hexanol/ether (40: 60, % v/v) and then back-extracted into a small volume of acidic aqueous solution before injection onto the HPLC column. With this procedure coefficients of variation were less than 8%. The detection limit was 0.5 μg/ml of urine. The method is simple, sensitive, selective and allows for routine analysis of urine samples in the genetic drug oxidation phenotyping study in ethnic population.