Rationale:Mucoepidermoid carcinoma (MEC) of the breast is a rare entity comprising specific morphological and immunohistochemical features, and has been previously only reported in 33 cases.Patient concerns:Four cases of MEC of the breast are reported in this study. All patients were women with ages ranging from 39 to 66 years. The lesions consisted of neoplastic solid nests and cystic spaces sometimes filled with mucoid material.Diagnoses:At high power, the tumors were composed of various proportions of basaloid, intermediate, epidermoid, and mucinous cells in different cases. All cases were classified as low-grade MEC of the breast. Tumor cells exhibited low levels of hormonal receptor expression in two cases (cases 1 and 3), and immunonegativity in one case (case 2). On the contrary, estrogen receptors (ER) were positively expressed in 60% of tumor cells in case 4. Tumor cells did not express human epidermal growth factor receptor 2 (HER-2)/neu protein in all the cases.Interventions:Modified radical mastectomy (Auchincloss) was performed in the first two cases, while the remaining two patients underwent mastectomy plus sentinel lymph node biopsy.Outcomes:All patients were alive and well without evidence of recurrent disease after a period ranging from 4 months to 156 months.Lessons:MEC of the breast is a rare primary carcinoma that is difficult to diagnose. Multiple tissue blocks are necessary before obtaining all cell types. Special stains for mucin and electron microscopy would be helpful in suspected cases. Hormonal factors might have an impact on the biological behavior of tumors, but further studies are needed to draw conclusions.