This study was undertaken to assess whether mild cervical intraepithelial neoplasia (CIN 1) lesions are histologically overdiagnosed and, if so, what the possible reasons are for this. The magnitude of the discrepancy between the histological diagnosis of CIN and corresponding smear results was also investigated. Between January and April 1988, 282 patients were found to have a histological diagnosis of CIN 1. The cervical smear result was negative in 42% of cases, borderline in 14% and in the remaining 44% various grades of CIN were predicted. Review of both cytology smears and histology sections was undertaken to identify reasons for this discrepancy. The results suggest that: (1) there is a tendency among histopathologists to overdiagnose cases of CIN 1, as occurred in 10% of cases in this study: (2) even if strict morphological criteria are followed, there will always be cases where histopathologists are uncertain whether a lesion is CIN 1 or some reactive process--for these cases, 22% in our study, the term borderline is recommended; and (3) cervical smears may give false negative results, as in 35%, 18% and 3% of our cases of CIN 1, CIN 2 and CIN 3 respectively.