Second primary breast cancer is of importance because of the increasing inc
idence of first primary breast cancer and improved survival. There is lack
of information on the disease in Canada. We studied 14,220 women with a fir
st primary breast cancer (invasive only) diagnosed during the period from 1
970 to 1997, in Manitoba, Canada. A breast cancer was considered to be a se
cond primary if it was an invasive lesion diagnosed at least six months aft
er the diagnosis of an initial primary breast cancer. The incidence rate of
second primary breast cancer was approximately six per 1,000 person-years,
and the cumulative incidence during the 28-year study period was 14.3%. Av
erage age at diagnosis of first primary breast cancer had increased signifi
cantly since 1970. The incidence was higher in 1975-1979, and there was no
significant difference among other study periods after adjustment for the a
ge at diagnosis of first primary breast cancer. Standardized incidence rati
os showed no significant difference among six age groups after adjusting fo
r calendar year. However, women with a first primary breast cancer had an i
ncreased risk of developing a second primary breast cancer compared with th
e risk of developing a first primary breast cancer among the general female
population. The SIR was markedly greater among women who had a first prima
ry breast cancer before the age of 40 years, and decreased significantly wi
th increasing age at diagnosis of first primary breast cancer. The data pro
vide epidemiological information on second primary breast cancer risk for t
he general female population in Manitoba, Canada.