To investigate clinical characteristics, treatment, outcomes, and prognostic risk factors of metachronous bilateral breast carcinoma (MBBC) and provide a theoretical basis for clinical management of MBBC.This was a retrospective study. From January 1, 2010 to March 31, 2022, a total of 23,010 patients with breast cancer underwent surgical treatment at the Breast Center of the Fourth Hospital of Hebei Medical University, including 386 patients with MBBC. Propensity score matching (PSM) was performed on MBBC patients and unilateral breast cancer (UBC) patients in a 1:1 ratio, and 210 UBC patients and 210 MBBC patients were finally matched. Clinical medical records of all patients were collected, including age of onset, family history of breast cancer, tumor size, lymph node status, TNM stage, mode of surgery, menstruation, pathological type, immunohistochemical (IHC) typing, treatment, disease-free survival (DFS), and overall survival (OS).The result showed that age of onset of the second primary cancer (SPC) was significantly older than that of the first primary cancer (FPC) (P = 0.024). Baseline data from MPPC patients showed that the tumor size of FPC was significantly larger than that of SPC (P = 0.043), and the proportion of PR ( +) in FPC is significantly higher than that in SPC (P = 0.045). Among MBBC patients with FPC for estrogen receptor (ER) or progesterone receptor (PR) ( +) and Her-2 (-), clinical characteristics and treatment results showed that the proportion of PR ( +) in the drug-resistant group was significantly lower than that in the non-drug-resistant group. The 2-year OS rate of SPC in the drug-resistant group was significantly shorter than those of the non-drug-resistant group (78.9% vs 100%, P < 0.05). The result of PSM-based comparison between MBBC patients and UBC patients showed significantly lower proportion of MBBC patients with SPC received chemotherapy compared to UBC patients (P = 0.026), and there was no significant difference in OS and DFS between SPC course of MBBC patients and UBC patients (P > 0.05). The univariate analysis showed that high TNM stage was a risk factor for death and disease progression in MBBC patients, with the risk of death in stage III MBBC patients being about 5 times higher than that in stage I MBBC patients (HR = 4.97, 95%CI = 1.42-17.31, P = 0.012), and the risk of disease recurrence being about 3.5 times higher than that in stage I MBBC patients (HR = 3.55, 95%CI = 1.07-11.81, P = 0.039).In summary, this study presented clinical characteristics, treatment options, and outcomes of MBBC patients and patients with MBBC who were resistant to endocrine therapy have a worse SPC survival prognosis. The course of SPC in MBBC patients was similar to that of UBC in terms of prognosis and survival, which suggested that SPC can be treated according to UBC treatment regimen. High TNM stage was a prognostic risk factor for SPC patients.© 2024. The Author(s), under exclusive licence to Federación de Sociedades Españolas de Oncología (FESEO).