Cyclophosphamide (CP), a widely used cytostatic, is metabolized by polymorphic drug metabolizing enzymes particularly cytochrome P450 (CYP) enzymes. Its side effects and clinical efficacy exhibit a broad interindividual variability, which might be due to differences in pharmacokinetics. CP-kinetics were determined in 60 patients using a global and a population pharmacokinetic model considering functionally relevant polymorphisms of CYP2B6, CYP2C9, CYP2C19, CYP3A5, and GSTA1. Moreover, metabolic ratios were calculated for selected CP metabolites, analyzed by (31)P-NMR-spectroscopy. Analysis of variance revealed that the CYP2C19*2 genotype influenced significantly pharmacokinetics of CP at doses