Mortality improvements in young women with breast cancer (BC) may be attributable to treatment advances; screening likely plays a less significant role as mammography is not recommended <40. We examined time-trends in outcome in a cohort of young women. Our goal was to determine the contributions of treatment and screening to mortality improvements and evaluate whether differential outcomes by ER status exist. Using SEER, patients (73,447) were divided into three categories by diagnosis year (1990-1994, 1995-1999, 2000-2004) and also categorized as <40 or 40-50 years. Multivariate analysis was done to investigate the association of survival with time period for both age groups by ER status. Hazard ratios (HR) for mortality in women 40-50 with ER positive BC declined over time. With 1990-1994 as referent, the HR in 1995-1999 was 0.77 (0.69-0.86) and 0.65 (0.59-0.71) in 2000-2004 (p < 0.001). Women <40 with ER positive BC also had improvements over time. In ER negative patients, the degree of improvements over time was less than that seen in ER positive women. We report a survival disparity over time in young women by ER status. Patients with ER negative disease have not had the degree of improvements over time as seen in ER positive disease. Therefore, mortality improvements in young women with ER positive BC may be attributed to treatment advances with endocrine agents. © 2013 Springer Science+Business Media New York.