Few studies have assessed the agreement between subjects' self-report and medical records among patients with breast cancer (BC), and none has addressed this issue in low-income women with BC. We assessed the level of agreement between self-report and medical records data for key BC treatment and prognostic characteristics using correct proportion and the Kappa statistic, among 726 low-income BC patients. Unconditional regression was used to investigate the association between accuracy of self-report and potential explanatory factors. Overall agreement between self-report and medical records was 95.3-99.6% for BC treatments including surgery, chemotherapy, radiotherapy and hormone therapy (Kappa = 0.79-0.99). Specific agreement was 87-89.5% for surgery type (Kappa = 0.51-0.96); 86.3% for chemotherapy completion (Kappa = 0.46) and 98.7% for radiotherapy completion (Kappa = 0.43); 95.2% for medical oncologist consultation (Kappa = 0.59) and 96% for radiation oncologist consultation; 97.3% for metastasis (Kappa = 0.56); and 93.6% for recurrence (Kappa = 0.30). When accepting answers within 15 days of the medical record date, 78.2% of women correctly reported surgery date, yet only around 55% of women correctly reported the start and/or end date of radiotherapy. Older age, less education, BC recurrence and poor patient-physician communication were associated with the lesser accuracy of patients' self-report compared to medical records (P < 0.05). The results of this study suggest that self-reporting of key treatment and prognostic information is relatively accurate among low-income women with BC. Self-report seems to be a reliable source for accurate information when medical record review is unavailable or unfeasible. Interventions to enhance patient-physician communication may facilitate more accurate information reporting among vulnerable populations.