Previous studies have argued that people feel regret more strongly when they act than when they fail to act (termed the action effect). This study investigated cultural differences in the circumstances under which the action effect occurs. Japanese and U.S. undergraduates played roulette games that determined either the participant's or a friend's outcome. After a computer randomly chose one of two roulettes, participants were asked to decide to change or not to change the roulette (i.e., action or inaction). Japanese participants regretted action more strongly than inaction under the friend condition, whereas U.S. participants showed the same pattern under the self condition. Additionally, those who regretted actions strongly were more likely to avoid actions. The authors discuss the functions of regret in culturally defined contexts.