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Background: In recent years, the survival of patients with breast cancer has improved. However, few published studies have a longer than 10‐year follow‐up. Conditional relative survival (CRS), which is relative survival (RS) of patients who have survived beyond a certain period after diagnosis, is useful for assessing excess mortality among long‐term survivors compared with the general population. Methods: This was a retrospective observational cohort study. Population‐based cancer registry data in Osaka, Japan were used to determine 15‐year RS and 5‐year CRS of women with breast cancer diagnosed between 2001 and 2002 and followed up for at least 15 years. Fifteen‐year RS and age‐standardized RS (ASR) were calculated by Ederer II and cohort methods. Five‐year CRS according to age group and extent of disease (localized, regional, and distant) was estimated for every year from diagnosis to 10 years. Results: In the cohort of 4006 patients, the ASR declined progressively, the 5‐year ASR being 85.8%, 10‐year ASR 77.3%, and 15‐year ASR 71.6%. The overall 5‐year CRS exceeded 90% at 5 years after diagnosis, reflecting a small excess mortality compared with the general population. The 5‐year CRS of patients with regional and distant disease did not reach 90% within 10 years of follow‐up (89.4% for regional and 72.9% for distant disease 10 years after diagnosis), indicating that these patients had substantial excess mortality. Conclusion: Long‐term survival data can help cancer survivors plan their lives and receive better medical care and support. [ABSTRACT FROM AUTHOR]