OBJECTIVE To evaluate the frequency of human papillomavirus (HPV) 16 and 18 infection in patients with different grades of cervical intraepithelial neoplasia (CIN). METHODS Five-hundred and five patients with CIN, referred for conization, were included in this study. Before conization, cytological material for in situ hybridization was obtained from the uterine cervix to detect the presence of HPV 16 and 18 infection. RESULTS Among all patients with CIN, 82 (16.2%) were solely HPV 16 and 51 (10.1%) were solely HPV 18 positive. There were 133 patients (26.3%) positive for HPV 16 or HPV 18 and 31 patients (6.1%) were positive for both viral types, giving an overall HPV 16/18 infection rate of 32.4%. There were 15 (55.5%) HPV 16 or HPV 18 positive patients with CIN 1, 45 (33.8%) HPV 16 or HPV 18 positive patients with CIN 2 and 104 (30.2%) HPV 16 or HPV 18 positive patients with CIN 3. CONCLUSIONS In patients with CIN 1, HPV 16 and 18 infection was more frequent than in patients with CIN 2, but the difference was not significant. Patients with CIN 2 were infected slightly more frequently, but not significantly, than patients with CIN 3. On the other hand, patients with CIN 1 were significantly more frequently infected than patients with CIN 3.