Background: Over the last decade, four epidemics of norovirus-associated gastroenteritis have been reported in Australia. These epidemics were characterized by numerous outbreaks in institutional settings such as hospitals and nursing homes, as well as increases in requests for NoV testing in diagnostic centers. During 2007 and 2008, widespread outbreaks of acute gastroenteritis were once again seen across Australia, peaking during the winter months.Objectives: The primary objective of this study was to characterize two winter epidemics of NoV-associated gastroenteritis in 2007 and 2008 in Australia. Following this, we aimed to determine if these epidemics were caused by a new GII.4 variant or a previously circulating NoV strain.Study design: NoV-positive fecal samples (n = 219) were collected over a 2-year period, December 2006 to December 2008, from cases of acute gastroenteritis in Australia. NoV RNA was amplified from these samples using a nested RT-PCR approach targeting the 5' end of the capsid gene, termed region C. Further, characterization was performed by sequence analysis of the RdRp and capsid genes and recombination was identified using SimPlot.Results: From 2004 to 2008, peaks in the numbers of NoV-positive EIA tests from the Prince of Wales Hospital Laboratory correlated with the overall number of gastroenteritis outbreaks reported to NSW Health, thereby supporting recent studies showing that NoV is the major cause of outbreak gastroenteritis. The predominant NoV GII variant identified during the 2007-2008 period was the GII. 4 pandemic variant, 2006b (71.51%, 128/179), which replaced the 2006a variant identified in the previous Australian epidemic of 2006. Four novel GII variants were also identified including the three GII. 4 variants: NoV 2008, NoV Osaka 2007 and NoV Cairo 2007, and one novel recombinant NoV designated GII.e/GII.12.Conclusion: The increase in acute gastroenteritis outbreaks in 2007 and 2008 were associated with the spread of the NoV GII.4 variant 2006b. (C) 2010 Elsevier B.V. All rights reserved.