Abstract Background The field of breast cancer care has experienced a rapid influx of new treatments, making it a potential focus for promotional activities by pharmaceutical companies. These financial relationships could turn into significant sources of conflict of interest within the healthcare sector, possibly leading to undue influence on breast cancer care. Despite the potential ramifications of these ties, there is a noticeable lack of understanding about the details of financial relationships between pharmaceutical companies and breast cancer specialists. The aim of this investigation was to understand the extent and distribution of financial relationships between pharmaceutical companies and breast cancer specialists, along with their trends over recent years in Japan. Methods Our study included all 1733 board-certified breast cancer specialists registered with the Japan Breast Cancer Society as of May 2021. We obtained data on the payments they received, along with the intended purpose of each payment, from 92 leading Japanese pharmaceutical companies between 2016 and 2019. This data was disclosed by the companies following the transparency guidelines set forth by the Japan Pharmaceutical Manufacturers Association. Firstly, we conducted a descriptive analysis of the extent and distribution of payments to specialists, including a calculation of the Gini index to illustrate the disparity of payments among these specialists. Secondly, we assessed the overall trends and individual-based payment patterns over a four-year period. Finally, we explored whether the introduction of new agents was associated with variations in payment values at the company level. Results In total, 64 companies made 11,774 payments, collectively amounting to 13,329,919 USD, with speaking fees comprising 81.4% of this total. Among the 1733 specialists, 1306 (75.4%) received at least one payment from these companies. The average payment per physician was 7,692 USD (with a standard deviation of 24,162), and the median payment was 2,044 USD (with an interquartile range from 798 to 5,207 USD). Among these specialists, 27 received payments of 100,000 USD or more. The Gini index was 0.99, indicating a significant disparity in payment distribution among the specialists. There was a rising trend in total payment values, from 2,883,954 USD in 2016, to 3,037,888 USD in 2017, 3,589,583 USD in 2018, and finally 3,820,041 USD in 2019. A notable increase in payment value occurred with Pfizer in 2018, following the launch of Palbociclib in December 2017. Similarly, Eli Lilly, which launched Abemaciclib in November 2018, also increased its payments to specialists from 2018 onward. Conclusion: The majority of board-certified specialists in Japan received payments from pharmaceutical companies between 2016 and 2019. However, these payments were concentrated among only a small fraction of the specialists. During the study period, the overall value of payments increased. This surge likely resulted from the launch of key drugs within the same class and with similar indications, specifically CDK4/6 inhibitors. Citation Format: Akihiko Ozaki, Kenji Gonda, Masahiro Wada, Hiroaki Saito, Erika Yamashita, Tetsuya Tanimoto. Nature and Trends of Pharmaceutical Payments to the Board Certified Breast Cancer Specialists in Japan between 2016 and 2019 [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 PO2-26-04.