Summary:Various cervical lesions at times may be difficult to distinguish from one another on routine hematoxylin and eosin stains, and immunostaining for the proliferation-associated antigen Ki-67, using monoclonal antibody MIB-1, can aid their distinction. The reduced MIB-1 expression in atrophy and increased MIB-1 expression in dysplasia permits easy distinction between these conditions. Presence of MIB-1 in more than 15% of basal cells and/or in surface half of the epithelium favor a diagnosis of condyloma over squamous metaplasia or inflammatory changes. Normal endocervix shows MIB-1 positivity in less than 10% of the cells, but usually in more than 20% of cells in cervical adenocarcinoma. With increasing grade of dysplasia, the percentage of MIB-1 positive cells is increased, and positive cells are seen in the higher levels of the epithelium. Presence of more than 20% MIB-1 positive cells in Pap smears showing atypical cells of uncertain significance is associated with a diagnosis of dysplasia on subsequent biopsies. Cauterized tissues with dysplasia show MIB-1 expression similar to adjacent noncauterized dysplastic areas. MIB-1 expression is, therefore, useful in evaluating various cervical lesions.