At present, data on the safety and efficacy of prasugrel in Japanese patients are limited to the populations of the phase II/III clinical trials, in which patients were selected based on strict inclusion criteria. In addition, because bleeding adverse events (AEs) have been reported as the most common adverse drug reactions (ADRs) in clinical trials, safety information should be made available in clinical settings as soon as possible by determining the incidence and severity of bleeding AEs under actual conditions of use. Therefore, this early postmarketing observational study (PRASFIT-Practice I) aimed to evaluate the safety and efficacy of the short-term use of prasugrel in patients with ACS in real-world clinical practice settings in Japan.