We conducted a two-way MR investigation using genome-wide association studies (GWAS) data to probe the possible causal link between gut microbiota and a range of breast diseases, including breast cancer [both estrogen receptor-positive (ER+) and estrogen receptor-negative (ER-) breast cancer], breast cysts, inflammatory disorders of the breast, and infections of the breast associated with childbirth. This evidence concerns the gene ESR1 and mastitis.