We determined L1 antibodies for human papillomavirus (HPV) types 6, 11, 16, 18 and 45 by multiplex serology in our prospective HPV family study. We report seroprevalence, seroconversion and antibody decay in 290 women (mean age, 25.5 years) sampled before delivery and at 12, 24 and 36 months of follow-up. Multiplex HPV genotyping of the baseline oral and genital scrapings was performed. At baseline, seroprevalence of HPV 6, 11, 16, 18 and 45 was 53.3, 21.5, 34.9, 21.5 and 9.0%, respectively. Seropositivity for low-risk HPV (LR-HPV) was associated significantly with age at onset of sexual activity (P=0.001), number of sexual partners until age 20 (P=0.018), lifetime number of sexual partners (P=0.0001), history of genital warts (P=0.0001) and being seropositive for high-risk (HR) HPV (P=0.0001). The same covariates also predicted seropositivity for HR-HPV. During follow-up, 26.7, 13.9, 17.0, 16.8 and 6.6% of the women seroconverted to L1 antigen of HPV 6, 11, 16, 18 and 45, respectively, between 18.2 and 23.8 months. Independent predictors of seroconversion to LR-HPV were unemployment (P=0.019) and absence of anal sex practice (P=0.031), and to HR-HPV, absence of smoking history and lifetime number of sexual partners. Decay of HPV 6, 11, 16, 18 and 45 antibodies was observed in 2.3, 4.0, 5.3, 4.5 and 1.5 % of the women, respectively, with decay time varying from 27.2 to 35.8 months. These data imply that (i) a substantial proportion of young women are seropositive for both LR- and HR-HPV types, (ii) they frequently undergo seroconversion within 18-24 months, predicted by common covariates, and (iii) antibody decay over 3 years is rare.