OBJECTIVE: To assess the safety of the quadrivalent human papillomavirus vaccine (HPV4) in females following routine administration. DESIGN: In a cohort of vaccinated females, we compared the risk of emergency department visits and hospitalizations during the interval soon after vaccination with risk during a comparison interval more remote from vaccination. SETTING: Kaiser Permanente in California. PARTICIPANTS: All females who received the HPV4 vaccine. MAIN EXPOSURE: One or more doses of HPV4 between August 2006 and March 2008. MAIN OUTCOME MEASURES: Outcomes were emergency department visits and hospitalizations, grouped into predefined diagnostic categories. Within diagnostic groups, we used odds ratios (ORs) to estimate whether each subject had any outcome in postvaccination risk intervals (days 1-60, days 1-14, and day 0), compared with a control interval distant in time from vaccination. RESULTS: One hundred eighty-nine thousand six hundred twenty-nine females received at least 1 dose and 44 001 received 3 HPV4 doses. Fifty categories had significantly elevated ORs during at least 1 risk interval. Medical record review revealed that most diagnoses were present before vaccination or diagnostic workups were initiated at the vaccine visit. Only skin infections during days 1 to 14 (OR, 1.8; 95% CI, 1.3-2.4) and syncope on day of vaccination (OR, 6.0; 95% CI, 3.9-9.2) were noted by an independent Safety Review Committee as likely associations with HPV4. CONCLUSIONS: The quadrivalent human papillomavirus vaccine was associated with same-day syncope and skin infections in the 2 weeks after vaccination. This study did not detect evidence of new safety concerns among females 9 to 26 years of age secondary to vaccination with HPV4.