A bioanalytical assay for the new poly(ADP-ribose) polymerase-1 inhibitor olaparib in combination with melphalan was developed and validated. For the quantitative assay, human plasma samples were pre-treated on ice using protein precipitation with 2% (v/v) acetic acid in acetonitrile containing erlotinib and melphalan-d 8 as internal standards. The extract was diluted with water and injected into the chromatographic system. This system consisted of a sub-2 μm particle, trifunctional bonded octadecyl silica column with an isocratic elution using 0.01% (v/v) of formic acid in a mixture of water and methanol. The eluate was transferred into the electrospray interface with positive ionization and the analyte was detected in the selected reaction monitoring mode of a triple quadrupole mass spectrometer. The assay was validated in a 10–5000 ng/ml calibration range for both drugs. The lowest level of this range corresponded to the lower limit of quantification. Within day precisions were 3.0–9.3%, between day precisions 6.0–9.8% and accuracies were between 101 and 110% for the whole calibration range. After validation the assay was used to assess the pharmacokinetics of olaparib in a patient with metastatic breast carcinoma. In addition, systemic exposure of melphalan was monitored in patients subjected to isolated hepatic perfusion with this drug. Both applications show that the new assay can be applied for human pharmacokinetic studies for both drugs.