Purpose To assess the risk of secondary nonbreast cancers (SNBCs) in a recently treated population-based cohort of breast cancer patients focused on the association with treatment and prognostic implications. Patients and Methods In 58,068 Dutch patients diagnosed with invasive breast cancer between 1989 and 2003, SNBC risk was quantified using standardized incidence ratios (SIRs), cumulative incidence, and Cox regression analysis, adjusted for competing risks. Results After a median follow-up of 5.4 years, 2,578 SNBCs had occurred. Compared with the Dutch female population at large, in this cohort, the SIR of SNBCs was increased (SIR, 1.22; 95% CI, 1.17 to 1.27). The absolute excess risk was 13.6 (95% CI, 9.7 to 17.6) per 10,000 person-years. SIRs were elevated for cancers of the esophagus, stomach, colon, rectum, lung, uterus, ovary, kidney, and bladder cancers, and for soft tissue sarcomas (STS), melanoma, non-Hodgkin's lymphoma, and acute myeloid leukemia (AML). The 10-year cumulative incidence of SNBCs was 5.4% (95% CI, 5.1% to 5.7%). Among patients younger than 50 years, radiotherapy was associated with an increased lung cancer risk (hazard ratio [HR] = 2.31; 95% CI, 1.15 to 4.60) and chemotherapy with decreased risk for all SNBCs (HR = 0.78; 95% CI, 0.63 to 0.98) and for colon and lung cancer. Among patients age 50 years and older, radiotherapy was associated with raised STS risk (HR = 3.43; 95% CI, 1.46 to 8.04); chemotherapy with increased risks of melanoma, uterine cancer, and AML; and hormonal therapy with all SNBCs combined (HR = 1.10; 95% CI, 1.01 to 1.21) and uterine cancer (HR = 1.78; 95% CI, 1.40 to 2.27). An SNBC worsened survival (HR = 3.98; 95% CI 3.77 to 4.20). Conclusion Breast cancer patients diagnosed in the 1990s experienced a small but significant excess risk of developing an SNBC.