Immunization with a vaccine of human papillomavirus (HPV) type 16 virus-like particles (VLPs) can reduce incidence of HPV-16 infection and its related cervical intraepithelial neoplasia. However, development of detectable antibodies to VLPs does not always occur after natural HPV infection. This study examined prospectively for seroconversion and duration of antibodies to HPV-16 VLPs and their associated host and viral factors. Six-hundred eight subjects were tested for HPV DNA biannually and for IgG and IgA antibodies to HPV-16 VLPs annually for 3 years. Both IgG and IgA antibodies to HPV-16 VLPs were predominantly type specific. Women with cervicovaginal HPV-16 infection were 8-10 times more likely to seroconvert than those with infection of HPV-16-related types. Among subjects who had an incident infection with HPV-16, a maximum of 56.7% became seropositive for IgG within 8.3 months and 37.0% had IgA within 14 months. Detectable seroconversion was a slow process that required sufficient antigenic exposure associated with either a high viral load (relative risk = 5.7 for IgG) or persistent infection of HPV-16 (relative risk = 3.4 for IgA). The median duration for both types of antibodies was approximately 36 months. Antibodies could persist for a long period of time if the initial antibody levels were high or if there was continued antigenic exposure.