The transitivity of osimertinib to cerebrospinal fluid (CSF) is of clinical concern. A quantitative LC-MS/MS method for the determination of osimertinib in human plasma and CSF was developed to evaluate its transitivity. The calibration range was 40-1000 nM in plasma and 0.8-100 nM in CSF. Accuracy and precision were within 15%. Osimertinib in the CSF but not in plasma strongly adsorbed onto the storage container. The mean adsorbed loss of osimertinib was 45.5% in CSF. Nonspecific binding in CSF was decreased by protein addition (mean loss = 5.8%). A robust validated method was developed for the quantification of osimertinib in human plasma and CSF.