The recent 2014 World Health Organization (WHO) Classification of Tumours of the Female Reproductive Organs introduced a new category of ovarian neoplasm designated "seromucinous tumours". The recognition of this distinctive group of tumors is an important addition to the classification but the term "seromucinous" has serious flaws that obscures the nature of these neoplasms. Morphologically, seromucinous tumors in addition to serous and endocervical-type mucinous epithelium, contain endometrioid, indifferent and squamous type epithelium. Their immunoprofile is characterized by frequent expression of ER, PR, infrequent expression of WT1 and lack of expression of CK20 and CDX2, an immunostaining pattern consistent with a "müllerian" immunophenotype. Unlike serous and intestinal type mucinous tumors, seromucinous tumors are frequently associated with endometriosis making them more analogous to endometrioid and clear cell neoplasms. Indeed, recent studies have shown that a high proportion of seromucinous tumors lost expression of ARID1A, a tumor suppressor gene, that is mutated in approximately 50% of endometrioid and clear cell tumors, in sharp contrast to serous and intestinal-type mucinous tumors which do not contain ARID1A mutations or lose its expression. Therefore, based on their clinicopathologic, immunohistochemical and molecular genetic features we believe a more appropriate designation for this group of tumors is "mixed müllerian tumors" which can be subcategorized as "mixed müllerian cystadenomas", "mixed müllerian atypical proliferative (borderline) tumors" and "mixed müllerian carcinomas".