The clinicopathological profiles of histiocytoid carcinoma of the breast have not been well examined because of their rarity and heterogenous groups of ductal and lobular origin. A large foamy or granular cytoplasm of histiocytoid carcinoma was characterized by abundant mucin, but the properties of mucin in histiocytoid carcinoma have also not been well investigated. We selected eight cases of histiocytoid features of invasive lobular carcinoma (HLC) and compared with 14 age- and tumor size-matched cases of classical invasive lobular carcinoma (CLC). Mucin profiles were significantly different between the two groups: a fair number of HLC cases were immunopositive for MUC2 and MUC5AC (75 and 50%, respectively); in contrast, almost all CLC cases showed both as negative. Both groups were immunopositive for MUC1 and negative for MUC4 and MUC6. The prognosis of HLC was significantly worse than CLC; HLC showed shorter disease-free time than CLC (p=0.0262). In particular, HLC with MUC2 and MUC5AC expressions showed significantly shorter disease-free time and survival time than lobular carcinoma without the expressions of MUC2 and MUC5AC (p=0.0055 and p=0.0060, respectively). Therefore, the expression of 'non-mammary mucins', such as MUC2 and MUC5AC in HLC, is characteristic and indicates the more malignant transformation of tumor cells and poorer prognosis.