The Danish Twin Register consists of the total population of twin pairs born in Denmark during a certain period. Within this population were found 50 monozygotic (MZ) female twins belonging to 45 MZ pairs and 81 dizygotic (DZ) twins belonging to 77 DZ female pairs, of which at least 1 twin had breast cancer (BC) and both twins were alive at the time of the first BC diagnosis. In 5 MZ and 4 DZ pairs, both partners had BC. Pairwise concordance rates were not significantly different between the 2 groups of twins but were of the same magnitude as in a previous study. The heritability, evaluated by genetic determination, was estimated to be 0.30-0.40. The observed number of BC cases developing in the co-twins after the first BC diagnosis in the twin pairs was increased by a factor of nearly 6 in MZ co-twins and by a factor of about 2 in DZ co-twins. For cancer of other sites, the observed and expected numbers were nearly identical in both MZ and DZ co-twins. In 8 of 9 pairs concordant for BC, the lesion was found on the same side. The mean age at diagnosis showed no significant difference between the concordant and discordant pairs. In the 40 MZ pairs with only 1 twin affected, she was more often unmarried and/or nulliparous than her unaffected, genetically identical co-twin, but this finding was not significant. A general tendency for the twin with BC to have the first child at a later age than her unaffected twin sister could not be demonstrated.