The pharmacokinetics of aprepitant, a neurokinin-1 receptor antagonist, have not been fully evaluated in clinical settings. The aim of this study was to characterize the plasma pharmacokinetics of aprepitant and reveal their influence of laboratory tests and cytochrome P450 (CYP) 3A5 gene polymorphisms in cancer patients. Forty-four Japanese cancer patients receiving cisplatin-based chemotherapy for the first time following oral aprepitant (125 mg on day 1 and 80 mg on days 2 and 3) were enrolled. The patients did not have gastrointestinal disease and the clinical laboratory values were within their normal reference levels. The plasma concentrations of aprepitant 24 (day 2 predose), 72, and 120 h after the first aprepitant administration were determined using LC-MS/MS. The relationships between plasma exposure to aprepitant and body weight, clinical laboratory values, age, gender, or CYP3A5*3 were investigated. The median and interquartile ranges of the 120-h area under the plasma concentration time curve ( AUC )_0–120 of aprepitant were 73215 and 55518–91121 ng h/mL. The coefficient of variation value for aprepitant AUC _0–120 was 53%. The AUC _0–120 of aprepitant was correlated with the levels of total bilirubin and serum albumin, respectively ( r =0.454, p <0.01 and r =0.287, p =0.06), but not with other non-genetic factors and CYP3A5 genetic variants in a univariate analysis. The AUC _0–120 of aprepitant was significantly correlated with the level of total bilirubin (adjusted R ²=0.187, p <0.01) in a multivariate analysis. In conclusion, the plasma pharmacokinetics of aprepitant varied markedly in cancer patients receiving cisplatin-based chemotherapy for the first time and were correlated with the level of total bilirubin.