Abiraterone acetate efficacy against prostate cancer is dependent on the circulating levels of abiraterone and its active metabolites, which present significant pharmacokinetic variability among patients. Thus, therapeutic drug monitoring can be performed to improve treatment outcomes. To support such studies, there is only a limited number of bioanalytical methods in current literature. This work presents a fast method to quantify abiraterone and D4A in plasma in 4 min by UPLC-MS/MS. Bioanalytical method validation was performed according to FDA recommendation. The method was linear within the range of 1-400 ng/mL for abiraterone and 0.2-20 ng/mL for D4A (r > 0.99). Based on the analysis of quality control samples at lower limit of quantification, low, medium and high concentrations, the method was precise (CV ≤9.72%; CV ≤14.64%) and accurate (CV 95.51-107.59%; CV 98.04-99.89%). Application of the method to the quantification of abiraterone and D4A in 10 clinical samples revealed important variability in the conversion ratio of abiraterone to D4A (CV 90.85%). Considering the current literature, this is the fastest method to quantify abiraterone and D4A in plasma, allowing for optimization of the analytical routine.This article is protected by copyright. All rights reserved.