Ki-67 nuclear antigen is present in proliferating cells. MIB-1 antibody, raised to the recombinant part of the Ki-67 antigen, is a widely used biologic marker to assess cell proliferation. Ki-67 expression is normally observed in parabasal and basal cells in the cervix. With increasing severity of dysplasia, MIB-1 labeling is seen in cells of the superficial layers of cervical epithelium, which are exfoliated. The purpose of this study was to determine the sensitivity and specificity of presence of MIB-1-positive cells in Papanicolaou tests for predicting cervical neoplasia, condyloma, or both, on follow up. Using microwave antigen retrieval method, 49 air-dried cervical smears in two-smear cases were evaluated with immunostaining with MIB-1 monoclonal antibody. Presence of MIB-1 positivity was arbitrarily set at > or = 4 MIB-1 immunoreactive cells in each smear. The degree of positive staining was correlated with the cytologic diagnoses, subsequent colposcopy-directed biopsies, endocervical curettage, and/or cytologic follow ups. Follow-up findings correlated with cytology in 33 cases (67%), with MIB-1 positivity in 35 cases (71%). Three cases with positive follow ups were missed by cytology but detected by MIB-1 staining, and three cases were missed by MIB-1 but detected by cytology. Both cytology and MIB-1 staining failed to detect a subsequent cervical lesion in two cases, and in six cases each, an abnormal finding was not substantiated on follow ups. MIB-1 immunostaining is a powerful technique for evaluating gynecologic smears and is as equally sensitive and specific as cervical cytology. It is able to identify cervical disease overlooked by cytologic screening; therefore, it may serve as an adjunct and complimentary tool to cervical cytology.