A reliable and easy to use liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed for the simultaneous quantification of urinary concentrations of cyclophosphamide (CP) and its main metabolites excreted in urine, i.e. N-dechloroethylcyclophosphamide (DCL-CP), 4-ketocyclophosphamide (4KetoCP), and carboxyphosphamide (CarboxyCP). Sample preparation consisted of dilution of urine with an aqueous solution of the internal standard D(4)-CP and methanol, and centrifugation. LC/MS/MS detection was performed using a triple-quadrupole mass spectrometer working in selected reaction monitoring mode. All analytes were quantified in a single run within 11.5 min. The limits of detection were 5 ng/mL for CP and 4KetoCP, 1 ng/mL for DCL-CP, and 30 ng/mL for CarboxyCP. Quantification ranges were adjusted to the expected concentrations in 24-h urine collections of patients treated with a polychemotherapy regimen (3-175 microg/mL for CP, 0.5-27 microg/mL for 4KetoCP and 0.17-9 microg/mL for CarboxyCP and DCL-CP, respectively). The method was validated according to international guidelines of the ICH and the FDA.