Persistent androgen synthesis under castration status in adrenal gland, testes and tumor cells is thought to be one of the major causes of development and progression of castration-resistant prostate cancer (CRPC). Abiraterone acetate (AA), the prodrug of abiraterone, which is an inhibitor of androgen synthesis enzymes, was evaluated for pharmacokinetics, pharmacodynamics, preliminary efficacy and safety in Japanese patients with CRPC in a phase-1, open-label and dose-escalation study. Chemotherapy-naïve Japanese CRPC patients (N = 27) received one of four AA daily doses (250 mg [n = 9], 500 mg [n = 6], 1000 [1 h premeal] mg [n = 6] and 1000 [2 h postmeal] mg [n = 6]) continuously through 28-day treatment cycles. In the first cycle, AA monotherapy was given on days 1-7 for pharmacokinetics, and AA plus prednisone (5 mg twice daily) from days 8 to 28. Of 27 patients, 9 continued treatment with AA until the data cut-off date (18 July 2013). Over the evaluated dose range, plasma abiraterone concentrations increased with dose, with median tmax 2-3 h. At each dose level, mean serum corticosterone concentrations increased, while testosterone and dehydroepiandrosterone sulfate concentrations rapidly decreased following a single AA dose and were further reduced to near the quantification limit on day 8 regardless of the dose. At least 3 patients from each dose-group experienced ≥50% prostate-specific antigen reduction, suggesting clinical benefit from AA in Japanese CRPC patients. AA was generally well-tolerated, and, therefore, the recommended AA dosage regimen in Japanese CRPC patients is 1000 mg oral dose under modified fasting conditions (at least 1 h premeal or 2 h postmeal). This study is registered at ClinicalTrials.gov: NCT01186484.