Cancer-related mortality can be measured by two dissimilar methods: cause-specific survival (based on mortality attributed to a specific cause), and relative survival (based on mortality relative to a matched cohort). We used both methods to determine actuarial survival in a population of 119,502 breast cancer patients from the Surveillance, Epidemiology and End Results (SEER) programme data set, with 20 years of follow-up. The population was divided into four strata by patient age and tumour stage. In all strata, there was only minimal deviation between the two survival methods. Of particular interest was the cause-specific treatment of patients recorded as dead of unknown cause, i.e. those deaths that could not be attributed with certainty to either 'breast cancer' or to 'other causes'. Findings suggest that the most reliable results may be obtained by apportioning these deaths between 'dead of cause' and 'withdrawn at the time of death'. This apportionment is based on the relative number of deaths attributed to 'breast cancer' versus 'other causes'.