We studied the incidence of microsatellite instability (MI) in lesions defined as cervical intraepithelial neoplasia (CIN). The human papillomavirus (HPV) status of the tissues was also determined. DNA from tissue samples and autologous lymphocytes were studied for five loci located within or adjacent to the DNA mismatch repair genes. Replicate errors were detected in 7 out of 47 (14.8%) samples of cervical tissue from 24 women. Our results indicate that the defect in DNA repair-associated genes does not appear to be necessary for the selection of clones which progress from dysplasia to carcinoma. Although HPV DNA of highly oncogenic types (16/18) was detected in most cervical lesions and may be an important factor for MI, we also detected MI in two loci in HPV-negative normal tissue, indicating that further events can also be involved in mismatch repair defects.