The gene numb encodes for a protein (Numb) involved in cell fate decisions in Drosophila, with proposed endocytic and developmental functions in mammalians. The distribution pattern of Numb in human tissues however, has not been fully characterized. We set out to explore the immunohistochemical expression of Numb in normal and neoplastic (28 adenoid cystic and 34 mucoepidermoid carcinomas) salivary glands, and correlated the results with the clinico-pathologic features of the neoplasms. Intense Numb immunoreactivity was detected in normal ductal cells and in a subset of acinar cells. In salivary carcinomas, we detected diffuse and intense Numb immunostaining in 5 adenoid cystic and 8 mucoepidermoid carcinomas. By contrast, the majority of adenoid cystic and mucoepidermoid cancers showed only moderate (14 and 5 cases) or focal staining (9 and 21 cases), respectively. The corresponding expression of Numb mRNA was documented in normal parotid gland and adenoid cystic carcinoma. Numb immunoreactivity was inversely correlated with the histological grade and Ki-67 immunoreactivity of both adenoid cystic and mucoepidermoid carcinomas. In addition, while tumor grade, stage, Ki-67 and Numb immunoreactivity were associated with disease-free survival in univariate analysis, only Numb and Ki-67 immunoreactivities retained independent prognostic significance in multivariate analysis. These data suggest that loss of Numb is implicated in aberrant differentiation programs of salivary gland carcinomas and may serve as a prognostic indicator in patients treated for these neoplasms.