In 175 women with mucinous carcinoma (MC) of the breast, the morphologic features, the clinicopathological features (age, tumor size, and nodal status) and prognoses were investigated. They were divided into two types, those with "unmixed type" (n = 140) showing no component of invasive ductal carcinoma (IDC) and "mixed type" (n = 35) including an IDC component. The unmixed type showed less frequent (P less than 0.01) nodal involvement and a higher degree (P less than 0.005) of extracellular mucus production than the mixed type. The presence of nodal metastases showed no correlation with the degree of mucus production. The 10-year survival rate of the unmixed type (90.4%) was better (P less than 0.001) than in the mixed type (66.0%). In the unmixed type, a higher level of mucus production showed trends toward a better prognosis, younger age and smaller tumor size (P less than 0.01). In the mixed type, the degree of mucus production showed no significant correlation with either the age, tumor size, or prognosis. It is more important for the prognosis of patients with MC to have no IDC component than to show abundant mucus within the tumor.