To study the temporal relationship between serum antibody response and human papillomavirus type 16 (HPV-16) infection, a cohort of 325 university women were scheduled for examinations at 4-month intervals. At every examination, interviews were completed, cells were obtained for polymerase chain reaction-based testing and for Pap screening, and serum was obtained for testing with a HPV-16 capsid-capture ELISA. Seroreactivity was associated with detection of HPV-16 DNA and with increased numbers of sex partners. The median time to seroconversion was 8.3 months among women with incident HPV-16 infections. Within 16 months following HPV-16 DNA detection, 93.7% of women with prevalent and 67.1% of women with incident infections seroconverted. After seroconversion, antibody responses were maintained during follow-up among HPV-16 DNA-positive women. Women who seroconverted were 5.7 times (95% confidence interval = 2.4-13.4) more likely to have squamous intraepithelial lesions associated with the detection of HPV-16 DNA than were women who did not seroconvert.