The social landscape surrounding hereditary breast and ovarian cancer (HBOC) in Japan has changed significantly since insurance coverage for part of its management began in April 2020. The introduction of poly(ADP-ribose) polymerase (PARP) inhibitors and their companion diagnostics has further expanded HBOC diagnosis, necessitating adaptation among healthcare providers unfamiliar with genetic diagnostics. In response, the Japan Organization of Hereditary Breast and Ovarian Cancer (JOHBOC) released the Guidelines for Diagnosis and Treatment of Hereditary Breast and Ovarian Cancer 2021 to standardize HBOC management. However, these guidelines, designed for medical professionals, were difficult for the general public to understand. To address this gap, JOHBOC and the Research on the Development of a Preemptive Medical Care Delivery System for Hereditary Tumors Using Genome Information group, supported by the Health and Labour Sciences Research Grant, developed a new guidebook tailored for patients and the public. A key feature of this guidebook is its emphasis on Patient and Public Involvement (PPI), a concept originally developed in the United Kingdom, which promotes collaboration with patients and the public in healthcare research and policy. The guidebook was created through extensive engagement with affected individuals, including patients, genetic counselors, and healthcare providers. A questionnaire survey collected input from 46 respondents, and four opinion-exchange meetings involving 36 participants were held. Ultimately, 82 members of the public contributed to the guidebook’s development, ensuring that the content was accessible and aligned with the needs of those affected by HBOC. To maximize its reach and usability, the guidebook was structured in a Q&A format, addressing common concerns and misconceptions about HBOC. A notable example is the discussion on communicating an HBOC diagnosis to minor children, which underwent multiple revisions based on public feedback to ensure clarity and practical applicability. Furthermore, efforts were made to make the guidebook affordable and portable, with a title and format designed to encourage widespread use. The development of this guidebook has also provided valuable insights for healthcare professionals, reinforcing the importance of clear, empathetic communication in genetic counseling. Feedback from both medical professionals and the public highlighted the guidebook’s role in fostering a shared understanding of HBOC, ultimately bridging the gap between medical knowledge and the lived experiences of patients and their families.