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Breast cancer mortality has been declining in many countries including Canada because of improvements in survival. This study attempts to explain observed trends in breast cancer survival with special attention given to the role of improvements in early detection and treatment.This study is based on 4,312 women diagnosed with primary invasive breast carcinoma treated in a Canadian breast center between 1976 and 2000 and followed to the end of 2001. Observed and relative survival rates were calculated. Multivariate relative survival regression models were used to assess trends in breast cancer survival over the study period.The proportion of women with small tumors (≤10 mm) was higher in late 1990s, while that of women with regional involvement was lower compared to earlier periods. Adjuvant chemotherapy or endocrine therapy use increased steadily from 6.6% to 84.0% during the study period. Five-year relative survival rates ranged between 82.1% and 83.7% between 1976 and 1990, and increased thereafter to reach 87.6% in 1991–95, and 92.1% in 1996–2000. During the first five years after diagnosis, women diagnosed in 1991–95 and 1996–2000 experienced a reduction in breast cancer mortality of 28% (Relative Risk (RR)= 0.72; 95% CI: 0.59–0.89) and 49% (RR = 0.51; 95% CI: 0.39–0.68) respectively compared to women diagnosed in 1976–90. Improvement in breast cancer survival in 1990's could not be explained by characteristics of women, biology of the tumor, advancements in early detection and type of initial treatments.A substantial increase in breast cancer survival was observed in the 1990s but the reasons for this improvement remain elusive. Better knowledge of these reasons could help not only to further reduce the burden related to breast cancer but also the burden related to other major cancer sites.