OBJECTIVES: To estimate the frequency of ambulatory care-sensitive hospitalizations (ACSHs) and to compare the risk of ACSH in breast cancer survivors living in high-poverty with that of those in low-poverty areas. DESIGN: Prospective, multilevel study. SETTING: National, population-based 1991 to 1999 National Cancer Institute Surveillance, Epidemiology, and End Results Program data linked with Medicare claims data throughout the United States. PARTICIPANTS: Breast cancer survivors aged 66 and older. MEASUREMENTS: ACSH was classified according to diagnosis at hospitalization. The percentage of the population living below the U.S. federal poverty line was calculated at the census-tract level. Potential confounders included demographic characteristics, comorbidity, tumor and treatment factors, and availability of medical care. RESULTS: Of 47,643 women, 13.3% had at least one ACSH. Women who lived in high-poverty census tracts (>or=30% poverty rate) were 1.5 times (95% confidence interval (CI)=1.34-1.72) as likely to have at least one ACSH after diagnosis as women who lived in low-poverty census tracts (<10% poverty rate). After adjusting for most confounders, results remained unchanged. After adjustment for comorbidity, the hazard ratio (HR) was reduced to 1.34 (95% CI=1.18-1.52), but adjusting for all variables did not further reduce the risk of ACSH associated with poverty rate beyond adjustment for comorbidity (HR=1.37, 95% CI=1.19-1.58). CONCLUSION: Elderly breast cancer survivors who lived in high-poverty census tracts may be at increased risk of reduced posttreatment follow-up care, preventive care, or symptom management as a result of not having adequate, timely, and high-quality ambulatory primary care as suggested by ACSH.