Abstract Background Breast cancer (BC) diagnosis and treatment can lead to multiple symptoms with a negative impact on the quality of life of survivors. Mindfulness-based stress reduction (MBSR) interventions have shown to improve anxiety, depression, and stress in patients with cancer across multiple settings. However, the effectiveness of an online MBSR program has not been studied. Methods This is an explanatory qualitative sub-study of a randomized trial of an online MBSR intervention versus a waitlist control. Participants were Mexican women aged ≥18 years diagnosed with stage 0-III BC 1-5 years prior to inclusion, not currently undergoing chemotherapy or radiotherapy, with moderate to severe anxiety (baseline GAD-7 score ≥10). The intervention consisted of 8 weekly 2-hour Zoom sessions with body scan meditation and yoga practice led by a certified instructor. Participants received a digital manual with exercises, audio for guided practice, and a video of postures for balance and flexibility. The primary outcome of the trial was anxiety at 2, 5, and 8 months after the intervention. Two focus groups were conducted via Zoom; 21 participants from the intervention arm were selected and invited according to the difference in GAD-7 scores between baseline and 8 months. The first group consisted of 7 participants (median age 53 years, range 39-62) with a decrease in GAD-7 score of ≥4 points (mean -9.1, SD 3.7); the second group consisted of 7 participants (median age 54 years, range 44-64) with a decrease of < 4 points or an increase in GAD-7 score (mean -1.2, SD 1.9). Each focus group was conducted 14 and 9 months after the MBSR intervention, respectively, and had a duration of 110 and 113 minutes, respectively. Audio and video were recorded and transcribed verbatim. Transcripts were analyzed and themes were developed, focusing on participants’ experience with the online intervention and its perceived benefits. Results The weekly sessions and at-home practice functioned as a “personal haven” for participants. At-home deliberate practice and the additional audio and video materials were considered essential for the effectiveness of the intervention. Participants no longer needed to consciously follow step-by-step directions, as the practices had become a part of who they were, even several months after the intervention. Breathing exercises were frequently used by participants on a day- to-day basis. Regarding the mindfulness practice itself, some participants indicated that they noticed positive changes in their character and demeanor, and in their reactions to everyday and BC-related stressors, including situations such as medical consultations and possible news regarding their treatment. In that sense, they mentioned that all they learned is useful for their life in general, and not exclusive to BC survivors. Despite differences in improvement of anxiety by GAD-7 scores between the focus groups, most participants in both groups referred to the intervention as “life-changing”. Most participants in both groups perceived the online format of the intervention as positive: it allowed them to join in from any location and fit the program into their daily routine with less time commitment. Likewise, the privacy of their homes made them feel more comfortable expressing themselves during the weekly sessions. A high level of recognition, appreciation and gratitude towards the instructor was observed. Some participants noted that contact with the instructor and other participants was limited due to the online format, preventing them from forming friendships or a sense of community. Conclusion BC survivors’ opinion of an online MBSR program was largely positive regarding the benefits of the intervention and its delivery format. Moreover, the intervention was successful at facilitating participants’ adoption of mindfulness practice in their daily lives and at improving other aspects of their lives beyond BC. Citation Format: Haydee Cristina Verduzco-Aguirre, Maura Pompa Mansilla, Misael Salazar-Alejo, Javier Gutierrez Ornelas, Fernanda Mesa-Chavez, Ana Ferrigno Guajardo, Cynthia Villarreal-Garza. Participants’ experience with an online mindfulness-based stress reduction intervention for anxiety management in breast cancer survivors: A secondary analysis of a randomized trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-11-09.