This double-blind, multicenter, randomized, placebo-controlled study was performed to test the efficacy of a bivalent HPV-16/18 L1 virus-like particle vaccine against human papillomavirus (HPV)-16/18 infection and associated cervical abnormalities, including precancerous lesions. Study participants, from 32 participating centers in Brazil, the United States, and Canada, were randomized to receive 3 doses of either vaccine or placebo delivered at 0 months, 1 month, and 6 months. Eligibility criteria included age 15 to 20 years, 6 or fewer sexual partners, no history of abnormal cervical smears or cervical treatment, no present treatment for condylomata, negative cytology, and seronegativity for HPV and HPV-18 antibodies and other high-risk HPV types. The vaccine was administered in 0.5-mL doses using monodose vials containing 20 μg each of HPV-16 and HPV-18 L1 virus-like particles. Placebo doses were administered in identical doses and were identical in appearance. Patients kept a record of dose reactions for 7 days after each injection. All participants who completed all components of the study were included in an “according-to-protocol” analysis. They were also included, along with those who had an incomplete record, in an “intention-to-treat” analysis. Analyses were performed at 18 and 27 months. A total of 1113 women participated in the study, 560 in the vaccine arm and 553 in the placebo arm. Both groups had similar demographic characteristics with an average age of 20 years. Incident HPV-16 and HPV-16/18 infections were significantly less common in the vaccine cohort at 18 and 27 months, but protection against HPV-18 did not reach significance in the according-to-protocol cohort. At 27 months in the according-to-protocol participants, there was 100% vaccine efficacy against persistent HPV-16 and HPV-16/18. Protection against persistent HPV-18 was significant in this cohort at 27 months but not at 18 months. In the intention-to-treat group, there was a highly significant difference in the number of cervical abnormalities between the 2 groups. Twenty-seven women in the placebo group had abnormal Pap smears compared with only 2 in the vaccine group (P <.0001). Similarly, in the according-to-protocol cohort, the vaccine efficacy was 93.5% (95% confidence interval 52, 3–99; P = .0002). HPV-51 and HPV-56 were detected at 9 months in 1 vaccinated woman. A cervical biopsy at 12 months showed CIN 1 and HPV-51, which persisted through month 21. In the placebo group, 3 participants developed CIN 1, and 2 developed CIN 2, all of which were associated with HPV-16. The vaccine group had significantly more pain, swelling, and redness associated with the injection (P = .0004). There were no other side effects associated with vaccination in either group. General symptoms such as fatigue, headache, itching, and so on, were equally common in both groups. Serology results showed that 100% of the vaccinated patients had converted to HPV-16- and HPV-18-positive by 18 months. The difference between the 2 groups for each antibody type was significant by month 7 (P <.0001). Compared with antibodies to HPV-16 and HPV-18, which developed as a result of naturally occurring infections, antibodies to HPV were 10 to 16 times greater in the vaccine group at the 18-month evaluation.