Thioridazine is metabolized in humans by CYP2D6 to mesoridazine, which is an active metabolite. Two or more CYP2D6 substrates are seldom given simultaneously to elderly patients because potentially dangerous metabolic interactions may occur. It may be valuable to know the CYP2D6 metabolic capacity of such patients to avoid drug interactions, which depend on the metabolic phenotype. The goal of this study was to evaluate the use of the mesoridazine/thioridazine ratio for the estimation of CYP2D6 enzyme capacity. A sensitive and reliable method has been developed for the determination of thioridazine and its metabolites, mesoridazine and sulforidazine. Commonly used central nervous system (CNS) comedications do not interfere with the method. A group of 27 chronic patients with mental illness receiving monotherapy with thioridazine were studied. There were 23 men and 4 women between 37 and 80 years old (mean +/- SD: 61.2 +/- 10.2). The thioridazine/mesoridazine ratio correlated with the debrisoquine metabolic ratio (r = 0.74, p < 0.001). Therefore, the authors suggest that the measurement of thioridazine and its metabolite might be a useful tool to assess CYP2D6 activity during treatment.