Survival rates for 67 women with bilateral breast cancer were compared to those for 1282 women with unilateral disease in a follow-up of 1349 women participating in a population-based study. Relative survival at 8 years of follow-up was 69% for women with unilateral disease as compared to 53% for women with bilateral cancer. When possible confounding histopathological differences--data about which were prospectively collected--and age were adjusted for in a multivariate analysis, the relative hazard rate was significantly higher for women with bilateral versus unilateral breast cancer (P = 0.006). The impact of interval time between the two primaries was analysed and a roughly two-fold higher hazard rate was seen for synchronous cancers with regularly falling risk for increasing interval times. This trend was however not statistically significant. The results indicate that the two tumours contribute independently to the patient's excess risk of dying and thus occur as two seemingly biologically unrelated events with respect to the tumour-host relationship and metastatic behaviour.