To evaluate predictors of contralateral breast cancer risk, we examined data from a nested case-control study of second primary cancers among a cohort of women in western Washington (United States) diagnosed with breast cancer during 1978 through 1990 and identified through a population-based cancer registry. Cases included all women in the cohort who subsequently developed contralateral breast cancer at least six months after the initial diagnosis, but prior to 1992 (n = 234). Controls were sampled randomly from the cohort, matched to cases on age, stage, and year of initial breast cancer diagnosis. Information on potential risk factors for second primary cancer was obtained through medical record abstractions and physician questionnaires. Women who were postmenopausal due to a bilateral oophorectomy (i.e., a surgical menopause) at initial breast cancer diagnosis had a reduction in contralateral breast cancer risk compared with premenopausal women (matched odds ratio [mOR] = 0.25, 95 percent confidence interval [CI] = 0.09-0.68), whereas no reduction in risk was noted among postmenopausal women who had had a natural menopause (mOR = 0.90, CI = 0.39-2.09). Among postmenopausal women, there was a suggestion of a lower risk associated with relatively high parity (2+). A family history of breast cancer was associated with an increased risk (mOR = 1.96, CI = 1.22-5.15) and varied little by menopausal status. Having an initial tumor with a lobular component (c.f. a ductal histology) was not related strongly to risk (mOR = 1.47, CI = 0.79-2.74). The results of the present and earlier studies argue that we have limited ability to predict the occurrence of a contralateral breast tumor. Better predictors will be required before diagnostic and preventive interventions can be targeted to subgroups of patients with unilateral breast cancer.