Caliciviruses are among the most common causes of gastroenteritis in people of all age groups. These antigenetically and genetically diverse viruses have been grouped into two genera within the family Caliciviridae, designated Norwalk-like viruses (NLV) and Sapporo-like viruses (SLV). To gain more insight in their epidemiology, we have developed a tentative genotyping scheme, which was used to differentiate the viruses detected in a set of epidemiological studies. NLVs and SLVs were detected by generic RT-PCR in stool specimens from 5.1% and 2.4% of cases with acute gastroenteritis for which a general practitioner was consulted, and in 16.5% and 6.3% of community cases of gastroenteritis. In addition, NLVs were associated with more than 80% of reported outbreaks of gastroenteritis from 1994-1999. Typically, several genotypes of NLV co-circulate in the community. Occasionally, however, several consecutive outbreaks were caused by essentially the same virus, although an epidemiological link had not previously been noted. This was most pronounced in 1995/1996, when a Lordsdale-like variant was detected that subsequently was found worldwide. This epidemic spread suggests differences in virulence or mode of transmission. In addition, we found that related NLVs are highly prevalent in calves in The Netherlands, raising questions about their potential for zoonotic transmission.