Human papillomavirus (HPV) infection causes cervical cancer and genital warts. Young women (1106) were randomized to receive one of three formulations of a quadrivalent HPV (Types 6/11/16/18) L1 virus-like particle (VLP) vaccine or one of two placebo formulations. The goal was to assess vaccine safety and immunogenicity in baseline HPV 6/11/16 or 18-naïve and previously infected subjects. All three formulations were highly immunogenic. At Month 2 (postdose 1), among women with vaccine-type antibodies at baseline, vaccine-induced anti-HPV responses were approximately 12- to 26-fold higher than those observed in baseline-naïve women, suggesting an anamnestic response. Following an initial, similar sized decline, anti-HPV responses plateaued and remained stable through end-of-study (3.0 years). No vaccine-related serious adverse experiences were reported.