Background: The impact of acetylsalicylic acid (ASA) on the clinicopathological characteristics of breast cancer has not yet been elucidated in detail; we therefore aimed to investigate the effects of ASA on the clinicopathological characteristics of patients with breast cancer. Patients and Methods: Patients diagnosed with breast cancer were retrospectively analyzed. Breast cancer patients who were taking ASA at the time of breast cancer diagnosis were enrolled as ASA users (n = 84); matching patients with the same age who were not taking ASA were included as control group (n = 890). Results: The median age was 56 (range 34-82) years in both groups. ASA users had a significantly lower incidence of grade II-III tumors compared to non-users (P = 0.02). The other clinicopathological characteristics and treatment histories were similar in both groups. In patients using ASA, the disease-free survival (DES) rate was 97.3%, 89.4%, and 79.9% and in non-users it was 94.1%, 81.8%, and 70.9% in the 1rst, 3rd, and 5th year, respectively (P = 0.01). In aspirin users, the overall survival rate was 95.0%, 90.6%, and 87.6% and in non-users it was 98.1%, 91.2%, and 85.5% in the 1rst, 3rd, and 5th year, respectively (P = 0.50). Conclusion: Using ASA at the time of breast cancer diagnosis was associated with significantly improved DES in breast cancer patients.