ObjectiveTo identify the factors associated with long-term regrets expressed a posteriori by randomized controlled trial (RCT) participants questioned about their decision to participate in an RCT. Study Design and SettingParticipants were questioned 6 years on average after their inclusion in a breast cancer adjuvant therapy RCT. Among 115 women from 21 centers, 93 (81%) answered a self-administered questionnaire based on the Decision Regret Scale (DRS). ResultsMean DRS score was 16.8 (standard deviation=15.9); 43.0% of participants expressed mild regret, and 25.8% expressed moderate to strong regret. A quarter of the women (25.6%) said that the decision was taken by the doctor alone, and 13.5% said it was not consistent with their own wishes.In the multivariate ordinal regression analysis, an involuntarily passive role in decision making was found to be associated with greater regret (cumulative proportional odds ratio=7.3, 95% confidence interval=2.0–27.6), regardless of age and being allotted or not to the standard treatment in the RCT. ConclusionWhether patients’ regret depended on their level of participation in the decision making or vice versa could not be determined in this cross-sectional survey, but efforts should be made to ensure that patients’ participation in trials is always based on an active personal decision.