Abstract Background: Breast cancer (BC) is the leading cause of cancer-related deaths among women worldwide. BC mortality rates have been decreasing over the past decades in the developed countries. In the United States, BC mortality declined on average 1.5% each year over 2008-2017. Given the paucity of BC mortality data in Brazil, we sought to characterize BC mortality trends in southeastern Brazil and its relationship with demographic variables. Methods: A cross-sectional registry-based analysis was conducted to describe BC mortality trends in the State of Sao Paulo (Brazil) from 2004 to 2017. Sao Paulo is the most populous state in Brazil, with 45.5 million inhabitants, corresponding to 21.8% of the country's total population. BC-related death records, including gender and age were collected from SEADE Foundation’s database, an official entity charged with generating statistical data for the State of Sao Paulo. Mortality rates are expressed in units of deaths per 100,000 individuals per year. The annual percentage change (APC) was calculated to identify mortality trends over the period. Trend analysis was carried out by linear regression and an increase or decrease in trend was considered statistically significant when p-value < 0.05. Results: From 2004 to 2017, 52,005 deaths from BC were recorded in the State of São Paulo, Brazil. Average annual mortality was 8.96/100,000 (17.38/100,000 for females and 0.11/100,000 for males). Average mortality rates were higher in the age group over 65 years-old (46.47/100,000). BC mortality rates remained stable during the period analyzed (APC 0, 95%CI -0.2% to +0.3%, P = 0.9) for both females (APC 0, 95%CI -0.3% to +0.2%, P = 0.7) and males (APC +2.4%, 95%CI -1.7% to +6.6%, P = 0.2). Among females, an increasing BC mortality trend was detected for patients under 40 years-old (APC +2.4%, 95%CI +1.6% to +3.2%, P < 0.0001) and remained stable for the age groups between 40 - 65 years-old (APC -0.3%, 95%CI -0.7% to +0.1%, P = 0.1) and over 65 years-old (APC -0.1%, 95%CI -0.4% to +0.2%, P 0.5). Conclusion: In contrast with data from other developing countries, a stable BC mortality has been demonstrated in southeastern Brazil over a 14-year period for the general population. However, a significant increase in BC mortality rates has been demonstrated among patients under 40 years-old. Citation Format: Guilherme Nader Marta, Laura Testa, Rodrigo Santa Cruz Guindalini, Gustavo Nader Marta, Maria Del Pilar Estevez Diz, Paulo M. Hoff. Trends in breast cancer mortality in Brazil - a 14-year registry-based study [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS7-27.