The incidence of a second primary breast cancer in the contralateral breast among 56,237 women with a first primary breast cancer diagnosed between the years 1943-80 in Denmark was established. The relative risk (RR) for a breast cancer patient to get yet another breast cancer was studied, taking account of age, stage and treatment of the first primary breast cancer. Based on 345,573 women years at risk and 1,840 non simultaneous contralateral breast cancer cases the overall relative risk (RR) of invasive cancer in the contralateral breast following a first primary breast cancer, was found to be 2.8 (95% Confidence Interval (CI); 2.7-3.0). Among women who survived 10 or more years the risk was higher among those irradiated for the primary breast cancer (RR = 2.6) than among non-irradiated (RR = 2.0). In the large group of patients with localized disease the association with radiation was obvious for all ages combined (irradiated RR = 3.0, not irradiated RR = 1.6), but not obvious among premenopausal (age less than 45 years) and perimenopausal (age 45-54 years) women at primary breast cancer until followed for 20 years. The RR was higher among irradiated than non-irradiated post-menopausal (age greater than 55 years) women from the time of diagnosis of the first cancer, but was not significant after 14 years of follow-up. The probability for a woman diagnosed with breast cancer at 45 years of age or younger, of developing a contralateral breast cancer if surviving to the age of 75 years, is 25%. Close surveillance of the remaining breast of breast cancer patients is advised, especially if young or following an irradiated localized primary breast cancer.