Norovirus (NV) (formerly called Norwalk-like virus) is the most common etiological agent of acute viral gastroenteritis outbreaks worldwide. Recent reports have shown that two new GII.4 variants caused epidemics in Europe. To investigate if it is also the case in Hong Kong, a molecular epidemiological study was undertaken between January 2002 and June 2005. During this period, there was a substantial increase in acute cases of gastroenteritis caused by NV. Phylogenetic analysis showed that GII.2 and GII.4 are the major circulating genotypes. Two new GII.4 variants (variants C and D) were identified in 2002 and 2004, which quickly became the predominant strains. They were almost identical to the variants causing epidemics in Europe recently. Since geographically distinct areas were involved within a short period of time, it is possible that GII.4 has a particular propensity for causing pandemics.