A case-control study was conducted to assess the risk factors associated with the development of a contralateral primary breast cancer among women who had had a first primary breast cancer. Hospital records were reviewed for 292 women with an incident contralateral breast cancer, diagnosed in one of eight hospitals between July 1, 1975 and December 31, 1983, and for a comparison group of 264 surviving unilateral breast cancer patients, previously diagnosed in the same hospitals. All subjects were identified through the records of the Connecticut Tumor Registry. Having an initial tumor containing lobular carcinoma was associated with an almost twofold increased risk of developing a contralateral cancer (aOR = 1.8; 95% CI: 1.0-3.5). Among those for whom a progesterone receptor assay was available, a positive assay was associated with an increased risk of a contralateral primary (aOR = 3.2; 95% CI: 1.0-9.5). AB blood type was also associated with an elevated risk, but this elevation was not statistically significant (aOR = 2.3; 95% CI: 0.7-7.7). Having received radiation treatment was not significantly associated with the risk of a contralateral primary (aOR = 0.9; 95% CI: 0.6-1.4), whereas chemotherapy treatment was associated with a significantly lowered risk (aOR = 0.3; 95% CI: 0.1-0.7). The association with chemotherapy appeared to be modified by body build (ROR = 1.5; 95% CI: 1.0-2.3 for a 2.5-unit differential in Quetelet's index).