Background: Women with hormone receptor-positive, early-stage breast cancer who adhere to adjuvant endocrine therapy (AET) reduce the risk of cancer recurrence and mortality. AET, however, is associated with adverse symptoms that often result in poor adherence. We applied participatory action research (PAR) principles to conduct focus groups and interviews to refine and enhance a web-enabled app intervention that facilitates patient-provider communication about AET-related symptoms and other barriers to adherence., Methods: We conducted four focus groups with women with early-stage breast cancer on AET (N=28), stratified by race (Black and White) and length of time on AET (<6 months and >6 months), to determine preferences and refine the app-based intervention. A fifth mixed-race focus group was convened (N=6) to refine THRIVE app content using high-fidelity mock-ups and to develop new, tailored feedback messages. We also conducted interviews with oncology nurses (N=5) who participated in the THRIVE randomized controlled trial., Results: Participants reported preferences for weekly reminder messages to use the THRIVE app, a free-text option to write in AET-related symptoms, and app aesthetics. Other requested app features included: a body map for identifying pain, sleep and dental problems on the symptom list, a dashboard, tailored feedback messages, and information about social support resources. Participants also developed new intervention messages, decided which messages to keep, and edited language for appropriateness and sensitivity. They also discussed the type of electronic pill monitor and incentive plan to be used in the intervention. Nurses reported THRIVE alerts integrated seamlessly into their clinical workflow and increased patient-provider communication, facilitating quicker response to patients' reported symptoms. Nurses reported no negative feedback or usability concerns with the app., Conclusions: THRIVE app content reflects researchers' partnership with a racially diverse sample of breast cancer survivors and healthcare providers and adherence to participatory design by incorporating patient-requested app features, app aesthetics, and message content. The app has the potential to improve AET adherence and quality of life among breast cancer survivors and reduce disparities in mortality rates for Black women by facilitating communication with healthcare providers., Competing Interests: Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/jhmhp-20-103). The series “Innovations and Practices that Influence Patient-Centered Health Care Delivery” was commissioned by the editorial office without any funding or sponsorship. Dr. GAV reports receiving personal fees from Roche/Genetech, Novartis, Eli Lilly, Immunomedics, Puma, Pfizer, AstraZeneca, Biotheranautics, Daiichi Sankyo, Vector Oncology and research funding from Roche/Genetech, Puma, Celcuity, Merck, BMS, Eli Lilly, GTx inc, Astrazeneca, Pfizer, Immunomedics, Tesaro, Halozyme, and ownership of Oncodisc, outside the submitted work. Dr. LS reports receiving personal fees from Amgen, Pfizer, Helsinn, Genentech, Genomic Health, BMS, Myriad, AstraZeneca, Bayer, Spectrum, Napo and research support from Amgen, Pfizer, outside the submitted work. Dr. IG reports receiving research support from Pfizer, outside the submitted work. Dr. RAK reports a speaking fee from General Mills, Inc, outside the submitted work. The authors have no other conflicts of interest to declare.