We have very limited information on serum neutralizing antibody in women naturally infected with the human papillomaviruses (HPVs) that are causally associated with cervical cancer. In this study, serum samples collected from 217 Japanese women with low-grade cervical intraepithelial neoplasia were examined for their neutralizing activities against HPV16, -18, -31, -52, and -58 pseudovirions. Eighty-four patients (39%), 35 patients (16%), 17 patients (8%), and 1 patient were positive for neutralizing antibodies against one, two, three, and four of these types, respectively. Presence of neutralizing antibody did not always correlate with detection of HPV DNA in cervical swabs collected at the time of blood collection. The neutralizing titers of the majority of sera, ranging between 40 and 640, were found to be conserved in the second sera, collected 24 months later, independently of emergence of HPV DNA in the second cervical swabs. The data strongly suggest that HPV infection induces anti-HPV neutralizing antibody at low levels, which are maintained for a long period of time.