The diagnosis of endocervical neoplasia can be difficult as it is sometimes mimicked by proliferative or reactive glands. MIB-1 is a proliferation marker that can aid in the diagnosis of squamous intraepithelial lesions (SIL) of the cervix and vulva, but its potential value in the diagnosis of endocervical lesions has not been fully explored. Ten formalin-fixed, paraffin-embedded cases of each of the following were obtained: morphologically normal endocervical glands from patients with cervical SIL, endocervicitis, microglandular hyperplasia (MGH), and endocervical adenocarcinomas (eight in situ, two invasive). Microwave unmasking of antigens was performed prior to immunohistochemical staining for MIB-1 using the avidin/biotin peroxidase method. Labeling indexes were calculated for 34 specimens (10 adenocarcinoma. 8 each of the other diagnoses) using image analysis (Samba 4000). There was diffuse MIB-1 reactivity in adenocarcinoma (labeling index 57-96%, mean 80%), minimal focal reactivity in normal glands underlying SIL (labeling index 0.8-4.3%, mean 2.4%), moderate spotty reactivity in MGH (labeling index 2.9-18.4%, mean 8.5%), and minimal to focally diffuse reactivity in endocervicitis (labeling index 1.0-13.3%, mean 5.7%). These data indicate that the percentage and distribution of MIB-1-reactive endocervical cells can be of diagnostic utility in distinguishing neoplastic glands from those of endocervicitis and MGH.