Background: Breast cancer is a common disease in radiation oncology. We evaluated the radiation dose received by breast cancer patients, an often-neglected concern. Materials and Methods: The total effective radiation dose in 101 breast cancer patients was calculated by summing the effective doses of individual diagnostic imaging tests from the first hospital visit to the initiation of radiotherapy. The effective dose from general radiography and computed tomography (CT) was estimated using tissue-weighting factors and dose-length products. The effective dose from isotopes (18F-fluorodeoxyglucose and 99m         Tc-methylene diphosphonate) was estimated from the radioactivity of each isotope using dose coefficients. The patient radiation exposures were analyzed using radiologic records in the Picture Archiving and Communication System. Results: The median duration from initial imaging to the initiation of radiotherapy was 4.5 months (range: 0.7–13.4 months). When comparing the average effective doses associated with each diagnostic modality, CT, positron emission tomography-CT, bone scanning and radiography occupied 64%, 21%, 10% and 5% of the total effective dose, respectively. Comparison of the total effective dose according to clinical factors (age, AJCC stage, T stage, N stage, operation method, and cancer location) by multivariate analysis revealed that only T stage was significantly correlated with the total effective dose (p = 0.004). The median total effective dose was 71.5 mSv (range: 11.9–131.9 mSv). Conclusion: The radiation dose received from diagnostic testing in breast cancer patients is not negligible. We need to systematically collect and manage the doses received by patients from medical procedures.