Mucoepidermoid carcinomas of salivary gland origin have an uncertain clinical course not directly predictable by histomorphology. The MIB 1 antibody, which detects Ki-67 antigen in formalin-fixed, paraffin-embedded tissues, was used to study cell proliferation in these tumors. An MIB 1 index was developed to express the percentage of MIB 1-positive proliferating cells, and the results were compared with histomorphological tumor grade and clinical outcome. All patients with MIB 1 indices lower than 10% in their primary tumors had a favorable clinical outcome. Most patients with MIB 1 indices higher than 10% developed a recurrent or metastasizing disease. All patients who died of their tumor or who had persistent tumor had MIB 1 indices higher than 10%. Thus, the MIB 1 index defines two virtually nonoverlapping forms of the disease, an indolent one and an aggressive one. Cell proliferation in mucoepidermoid carcinomas, assessed with the MIB 1 antibody, thus represents a significant prognostic factor for improving the accuracy of conventional histological grading.