BACKGROUND: The association between common breast cancer therapies and recurrences and second primary breast cancers in older women is unclear, although older women are less likely to receive common therapies. METHODS: Women aged >or=65 years who were diagnosed with stage I or II breast cancer and who underwent mastectomy or breast-conserving surgery (BCS) from 1990 to 1994 were identified from automated data from 6 healthcare systems and then were followed for 10 years or until breast cancer recurrence, disenrollment, or death. Trained abstractors reviewed medical records to obtain recurrence, tumor, treatment and demographic data. The authors used proportional hazards models to examine predictors of recurrent and second primary breast cancers adjusted for demographic and tumor factors. RESULTS: Of 1837 eligible women, 34% were ages 65 to 69 years, 46% were ages 70 to 79 years, and 20% were aged >or=80 years. In multivariable models that used mastectomy as the reference group, BCS without radiation therapy was associated with an increased risk of any recurrent and second primary breast cancer (hazard ratio [HR], 1.6; 95% confidence interval [CI], 1.1-2.3), particularly with the subgroup of women with local and regional recurrence (HR, 3.5; 95% CI, 2.0-6.0). Tamoxifen use for <1 year versus >or=5 years exhibited a borderline association with any recurrent or second primary breast cancer (HR, 1.9; 95% CI, 0.9-4.2). CONCLUSIONS: Radiation therapy after BCS and 5 years of tamoxifen use were beneficial in reducing recurrences and second primary breast cancers in older women, regardless of their age or comorbidity burden.