BACKGROUND: In 7 large managed care organizations (MCOs), we performed a post-licensure safety assessment of quadrivalent human papillomavirus vaccine (HPV4) among 9-26 year-old female vaccine recipients between August 2006 and October 2009. METHODS: Sequential analyses were conducted weekly to detect associations between HPV4 exposure and pre-specified outcomes. The pre-specified outcomes identified by ICD-9 codes using computerized data at the participating MCOs included: Guillan-Barre Syndrome (GBS), stroke, venous thromboembolism (VTE), appendicitis, seizures, syncope, allergic reactions, and anaphylaxis. For rare outcomes, historical background rates were used as the comparison group. For more common outcomes, a concurrent unexposed comparison group was utilized. A standardized review of medical records was conducted for all cases of GBS, VTE, and anaphylaxis. RESULTS: A total of 600,558 HPV4 doses were administered during the study period. We found no statistically significant increased risk for the outcomes studied. However, a non-statistically significant relative risk (RR) for VTE ICD-9 codes following HPV4 vaccination of 1.98 was detected among females age 9-17 years. Medical record review of all 8 vaccinated potential VTE cases in this age group revealed that 5 met the standard case definition for VTE. All 5 confirmed cases had known risk factors for VTE (oral contraceptive use, coagulation disorders, smoking, obesity or prolonged hospitalization). CONCLUSIONS: In a study of over 600,000 HPV4 vaccine doses administered, no statistically significant increased risk for any of the pre-specified adverse events after vaccination was detected. Further study of a possible association with VTE following HPV4 vaccination is warranted.