The Cancer Registry, which was initiated in 1942, is regarded as virtually complete and contains diagnoses classified by an extended Danish version of the International Classification of Diseases, revision 7, and date of diagnosis. In total, 218 women who had had breast cancer (66%) were still alive and living in Denmark at the time of interview (between March 2005 and December 2006). We received mandatory permission from the treating hospitals to contact 210 (96%) of these live cases. We chose 899 live potential female controls at random from the cohort by incidence density sampling and approximately frequency matched (1:4) them with cases on the distribution of 1-year groups of birth years. The current address of each potential participant was retrieved from the Central Person
Register, which is updated daily.