Cyclophosphamide is an alkylating agent commonly used in chemotherapy regimens for breast cancer, non-Hodgkin's lymphoma, leukemia, and lung cancer. However, the use of cyclophosphamide can cause toxic side effects on the bladder, namely hemorrhagic cytitis which can cause hematuria and can later develop into bladder cancer. These side effects are caused by the byproduct of cyclophosphamide metabolism, acrolein. In this study, we developed a method to quantify 3-Hydroxy Propyl Mercapturic Acid (3-HPMA) in cancer patient’s urine. 3-HPMA is a stable metabolite of acrolein that serves as biomarker of acrolein. Urine samples were taken 4 hours after cyclophosphamide administration and urinalysis was done to observe the risk of hematuria. Analysis of 3-HPMA was performed by reversed phase UPLC-MS/MS equipped with triple quadrupole mass spectrometer positive ESI mode detection. The mobile phase used for analysis is 0,1% formic acid in water and in acetonitrile (90:10 v/v). The MRM was set at m/z 222.10>90.97 for 3-HPMA and 164.10 > 122.02 for the internal standard NAC. Sample preparation was done by acidification and simple dilution.The analytical method used is linear within the consentration range of 40-10000 ng/mL. The results showed varied levels of 3-HPMA in 40 cancer patients urine, with measured concentrations ranging from 113-9495 ng/mL and creatinine-adjusted levels ranging from 650-5596 ng/mg creatinine. Patients with positive results of hematuria showed 3-HPMA levels that were relatively high with mean level of 4327 ng/mg creatinine.