OBJECTIVES:To determine the relative incidence (RI) of Guillain-Barré syndrome (GBS) in a single Australian state following pandemic (H1N1) 2009 influenza A immunisation (monovalent vaccine or seasonal trivalent influenza vaccine [TIV]) in 2009-2010. DESIGN, SETTING AND PARTICIPANTS:Active GBS surveillance (cases assessed by two neurologists according to the Brighton criteria) from 30 September 2009 to 30 September 2010, conducted at 10 hospitals in Victoria, Australia. MAIN OUTCOME MEASURES:The RI of GBS in the risk window of 0-42 days after vaccination. RESULTS:Sixty-six potential GBS cases were identified, with complete data on 50 confirmed cases. The Victorian annual incidence of GBS was 1.7 per 100 000 population. Three cases had received monovalent vaccine and one case had received seasonal TIV within 42 days of symptom onset. The RI of GBS following monovalent vaccination was 3.4 (95% CI, 0.8-15.0). For TIV, there was one case in the risk period (RI, 0.69; 95% CI, 0.08-5.64). CONCLUSIONS:This is the first published study reviewing GBS after a trivalent and/or monovalent influenza vaccine containing the pandemic (H1N1) 2009 strain, with only a small proportion of GBS cases occurring after influenza immunisation. H1N1-containing vaccines were not statistically associated with GBS, but this study could not exclude smaller increases in the RI. Active surveillance of adverse events following immunisation is required to maintain public and health care professional confidence in mass vaccine implementation programs.