Several earlier studies have assessed survival in breast cancer based on familial risk of this disease. The results have been conflicting and suggest that the risk and prognostic factors of cancer are largely distinct. As a novel concept, we searched for familial clustering of survival, i.e., concordance of survival among family members. We used the nation-wide Swedish Family-Cancer Database to estimate hazard ratios (HRs) for cause-specific and overall survival in invasive breast cancer. HR shows the probability of death in the study group compared the reference group. The study covered 1277 mother-daughter pairs with familial breast cancer. Their median follow-up times for survival ranged from 96 to 122 months. When the survival in daughters was analyzed according to the mothers' length of survival, there was a concordance of prognosis. The HR was 0.65 in daughters whose mothers had survived > or = 120 months compared to daughters whose mothers had survived less than 36 months (P-value for trend 0.02). When the analysis was reversed and HRs were derived for mothers, the results were essentially similar (P-value for trend 0.02). The survival did not differ between patients with familial or sporadic breast cancer. The results are consistent in showing that both good and poor survival in breast cancer aggregates in families, which is a novel population-level finding for any cancer. The consistency of the results suggests that the prognosis in breast cancer is in part heritable which is likely to be explained by yet unknown genetic mechanisms.