Mentally depressed breast cancer (MDBC) patients expressed estrogen receptor (ER) and 16α‑hydroxyestrone (16α‑OHE1) is directly responsible for causing breast cancer (BC). This study aimed to identify whether depression in breast cancer patients enhanced the production of autoantibodies against 16α‑OHE1-ER adduct in breast cancer patients. The antibodies in the serum of 65 breast cancer patients (including 35 MDBC) and 40 control subjects were screened by direct binding, inhibition enzyme-linked immunosorbent assay (ELISA), and quantitative precipitin titration. Competition ELISA was also utilized for the estimation of 16α‑OHE1 in the serum of 30 cancer patients. Autoantibodies from MDBC showed strong recognition to 16α‑OHE1-ER in comparison to overall breast cancer patients (p