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Background; Many studies have demonstrated an association between diabetes and breast cancer risk. Purpose; We examined whether fasting glucose and hemoglobin A1c (HbA1c) are associated with mammographic breast density in Japanese women. Methods; Study consisted of 6,203 women who attend breast cancer screening with the medical health check-up (2014-2019). We used mammographic breast density assessed at the first breast screening. Fasting serum glucose and HbA1c were examined at the same day on the breast cancer screening. The association between breast density and fasting serum glucose and HbA1c were analyzed by logistic regression adjusted for potential confounding factors. Serum glucose and HbA1c levels were calculated with continuous variables. BI-RADS c and d were defined as dense breast tissue. Results; Of 6,203 women with mean age of 51 years, dense breast tissue was present in 60.0 % of participants. 960 (15.1%) had hyper-glycemia (≥100) and 2,830 (45.62%) had hyper-HbA1c (>5.5%). As table 1 illustrates, hyper-glycemia and hyper-HbA1c were significantly inversely associated with dense breast tissue, (serum glucose: odds ratio; 0.987 confidence interval: 0.982-0.992, p Citation Format: Ken Uchida, Naruki Kitano, Daisuke Yamaguchi, Hiroko Nogi, Makiko Kamio, Hiroshi Takeyama. Impaired glucose tolerance decreases mammographic breast density in Japanese women [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-08-24.