The incidence of breast cancer rises steeply between ages 25 and 50, and mo
re slowly thereafter. In contrast, the incidence in the unaffected (contral
ateral) breast of women who have had breast cancer remains constant at abou
t 0.7% per year for at least the next 20 years after diagnosis, irrespectiv
e of age at first diagnosis. The incidence in relatives of the patients see
ms to show a similar pattern. The incidence in a prospective study of monoz
ygotic twins of patients was approximately constant at 1.3% per year (77 ca
ses), again about 0.7% per breast. At ages older than a patient's age at di
agnosis, her mother and sisters have an incidence of 0.3-0.4% per year. Abo
ve the index patient's age at diagnosis, the rate in relatives shows no tem
poral trend and is independent of the patient's age at diagnosis. A statist
ically simple explanation is that incidence in susceptible women increases
to a high constant level by a predetermined age that varies between familie
s, but this seems inconsistent with conventional models of carcinogenesis a
nd susceptibility. The very high incidence in monozygotic twins of patients
indicates that a high proportion, and perhaps the majority, of breast canc
ers arise in a susceptible minority of women.