Background: Prophylactic vaccination with a quadrivalent HPV (types 6, 11, 16, 18) vaccine (qHPV) has been shown to prevent infection with HPV 6/11/16/18 and associated disease in women and more recently, in men. Here we report on the safety and reactogenicity of the qHPV vaccine in males. A total of 4,065 healthy males aged 16-26 years were enrolled into a randomized, placebo-controlled, double-blind trial. Subjects were randomized 1:1 to receive qHPV vaccine or placebo at day 1, month 2, and month 6. Safety and tolerability were assessed via the collection of reported adverse experiences (AEs). All serious AEs (vaccine- or procedure-related or not) and all deaths occurring during the study were recorded. Safety analyses were conducted in all subjects who received at least one dose of vaccine or placebo. The proportion of subjects who reported at least one injection-site AE was higher in the qHPV vaccine group versus the placebo group (60.1% vs 53.7%, respectively), however most of these AEs were mild/moderate in intensity. The incidence of at least one systemic AE was comparable between the vaccine and placebo groups (31.7% vs 31.4%, respectively). There were no vaccine-related serious AEs or deaths. The occurrence of AEs did not increase with each successive injection, and among trial participants who were seropositive for at least one vaccine HPV type at enrollment, the profile of adverse events was similar to that of the entire study cohort. The qHPV vaccine was generally well tolerated in males aged 16-26 years and had a favorable safety profile.