The immunohistochemical expression of p16INK4a in formalin-fixed and paraffin-embedded histological sections was evaluated in a retrospective study comprising a low-grade group of 100 cases of cervical intraepithelial neoplasia (CIN) 1, a high-grade group of 50 cases of CIN 2 to 3, and a benign group of 50 cases of normal tissue or benign lesions in the uterine cervix. The cases were consecutive within each group and had a minimum follow-up period of 5 years. Positive reaction for p16INK4a was detected in all cases in the high-grade group and in only 3 cases in the benign group. In the low-grade group, a total of 9 cases had to be excluded. The remaining 91 cases in the low-grade group showed positive reaction for p16INK4a in 65 cases (71%), including 23 cases that progressed to a high-grade lesion, 36 cases that revealed normal cytological and/or histological picture during the follow-up period, and 6 cases that persisted as CIN 1. A total of 26 cases (29%) in the low-grade group showed negative reaction for p16INK4a. All but one of these p16INK4a negative cases in the low-grade group had a benign or normal outcome. This case showed a high-grade lesion in the follow-up period and was probably a high-grade lesion from the beginning and so underestimated as CIN 1. These results reveal that the negative predictive value of p16INK4a to predict the outcome of the cases of CIN 1 is as high as 96%, which strongly suggest an important role of p16INK4a in the assessment of this type of lesion.