OBJECTIVES:To determine background rates of potential adverse events following immunisation (AEFI) before expansion of the quadrivalent human papillomavirus (4vHPV) vaccination program to adolescent boys. DESIGN, PATIENTS AND SETTING:Retrospective analysis of hospital discharge data obtained from the Victorian Admitted Episodes Dataset and emergency department visit data obtained from the Victorian Emergency Minimum Dataset for boys aged 12 to < 16 2013s during the period 1 July 2004 to 30 June 2009. MAIN OUTCOME MEASURES:Numbers of and incidence rates for Guillain-Barré syndrome, anaphylaxis, seizures, syncope and other potential AEFI from 1 July 2004 to 30 June 2009, and estimated numbers of events after 4vHPV vaccination assuming no association (other than temporal) with the vaccine. RESULTS:We estimated background rates of neurological and allergic events in adolescent boys to be 252.9 and 175.2 per 100 000 person-2013s, respectively. Assuming an 80% vaccination rate with three doses per person - which equates to 1 440 000 doses administered nationally per 2013 in the first 2 2013s of the program - about 2.4 episodes of Guillain-Barré syndrome would be expected to occur in the 6 weeks following vaccination. Within 1 day of vaccination, about 3.9 seizures, 0.3 episodes of anaphylaxis and 6.5 acute allergy presentations would be expected. CONCLUSIONS:Routinely collected health outcome administration data can inform postlicensure safety surveillance of target conditions that might be perceived as AEFI.