Identifying individuals with a deficient capacity for oxidative drug metabolism is of increasing clinical importance. Dextromethorphan (DM) is gaining wide acceptance as a probe drug to characterize individual expression of a specific cytochrome P-450 isozyme. The thin-layer chromatography (TLC) technique described in the present study is a rapid and inexpensive alternative to the methods currently available for assessing the urinary metabolic profile of DM. Sixty-five healthy volunteers participated in the study by ingesting 213 mmol DM and collecting all urine for the ensuing 8 h. Urine samples were analyzed by TLC and high-performance liquid chromatography (HPLC) after treatment with beta-glucuronidase. Based on the relative color intensities of DM and its O-demethylated metabolite, dextrorphan, the TLC analysis provided an accurate phenotype assessment. A greater intensity of the parent drug relative to the metabolite indicates a poor metabolizer phenotype whereas a reversed relative intensity indicates the extensive metabolizer phenotype. The phenotype assignments made by TLC were verified by comparison with the quantitative results (based on metabolic ratios) obtained from HPLC analysis. Complete agreement was found between the two methods. The routine implementation of phenotype determination into clinical protocols can be realized with this facile TLC technique.