The incidence of breast cancer among 1240 women who were treated surgi
cally for breast hypertrophy in Copenhagen, Denmark between 1943 and 1
971 was determined and compared with age- and calendar period-specific
rates for the Danish female population. A total of 32 cases of breast
cancer had developed by the end of 1990; the expected number was 52.5
5, yielding a relative risk (RR) of 0.61 [95% confidence interval (CI)
0.42-0.86]. The greatest reduction in risk was observed for women who
had 600 g or more of breast tissue removed (RR=0.30; 95% Ct 0.10-0.69
). This suggests that the number of potential foci is important for ca
ncer development in the female breast. In the group of women who were
operated on before the age of 20, four cases of breast cancer develope
d, compared with 2.23 expected cases, to give an RR of 1.79, suggestin
g that the aetiology of their breast hypertrophy may be different from
that for the rest of the group.