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Previous reports on the influence of the sex of a child and prognosis of a subsequent breast cancer have been conflicting. We took advantage of a number of large and good quality, nationwide Registries in Sweden to evaluate the prognostic value of the sex of the first child in breast cancer. A population-based cohort of 32,003 women born after 1935 and diagnosed with primary breast cancer between 1958 and 1997 was generated by linking a number of Swedish registries including Swedish Cancer Registry, Cause of Death Registry, Swedish Generation Registry and the Registry of Population and Population changes. Of these, 4,251 were nulliparous, 2,732 had only a boy and 2,497 had only a girl prior to diagnosis. We used this subpopulation to quantify the association between the sex of the first child and breast cancer-specific mortality, using the Cox proportional hazards. Sex of the child did not significantly influence the prognosis of breast cancer. Stratifying by age and adjusting for age at diagnosis, calendar period, time since last birth and age at first child did not alter the results. Our study, the largest up to date, failed to identify evidence to support the proposed association between the sex of the first child and breast cancer survival. Given the previous reports that advocated taking sex of the first child into consideration in making decisions related to breast cancer therapy, our findings are reassuring and clinically important.