Breast cancer laterality was studied in relation to age in 80,784 case
s of invasive and 3,835 cases of pre-invasive breast cancer in women a
nd 548 cases of invasive breast cancer in men reported to the Swedish
Cancer Registry, 1970-89. In a subset of 11,274 women with invasive di
sease, data on parity were available through the Swedish Fertility Reg
istry. Laterality also was evaluated in relation to age and reproducti
ve variables in 3,986 cases from an international study from the 1960s
. The overall incidence of pre-invasive and invasive cancer was higher
in the left than in the right breast among both women and men. The ex
cess incidence of invasive cancer in the left breast was evident only
after the age of 45 years in women; a similar phenomenon may exist wit
h pre-invasive disease in women and in men. The age-dependent laterali
ty pattern did not appear to be confounded by menopausal status. Among
women younger than 45 years, nulliparity, right handedness, and late
age at menarche was associated with a somewhat higher incidence of can
cer in the right breast. The laterality findings are likely to be due
to factors operating early in the carcinogenic process, perhaps at the
pre-initiation stage.