A series of 47 lesions diagnosed cytologically as cervical intraepithelial neoplasia (CIN) III in 22 cases, CIN I/II in 13 cases, and 12 cases showing abnormal smears consistent with human papillomavirus infection were analyzed. Thirty-six cases with negative cytology were used as the control group. Sixty-eight percent of CIN III were positive for a mixture of human papillomavirus 16 and human papillomavirus 18, 18% reacted with human papillomavirus 6 or 11, and 14% were negative. Of the group with CIN I/II or with abnormal Papanicolaou smears, approximately one-third contained human papillomavirus 6 (11) and one-third human papillomavirus 16 and 18. Only 11% of the samples from the control group hybridized with human papillomavirus 6 (11), the others were negative with either probe. The data obtained by the rapid in situ hybridization of cervical cells are in agreement with the presence of human papillomavirus 16 and 18 in a high proportion of cervical carcinoma and carcinoma in situ lesions. Thus, the method can be applied to test the hypothesis that a lesion containing human papillomavirus 16/18 positive cells has a higher risk of progressing to cancer than a lesion harboring human papillomavirus 6 or 11.