[Abstract] Objective To study the epidemiology of viral diarrhea among children <5y in China. Methods Hospital-based study in 10 sentinel hospitals, community-based study in two rural counties, and strain surveillance in other 8 provinces were conducted during 1998-2005. Fecal specimens were collected and tested for rotavirus, astrovirus, adenovirus respectively by using enzyme immunoassay (IDEIA Rotavirus, Amplified IDEIA Astrovirus, and IDEIA Adenovirus Kit, Dako, United Kingdom), RT-PCR and a multivalent EIA for HuCV detection. RT-PCR and sequencing for virus characterization. Results Rotavirus was the most common etiologic agent identified among children hospitalized for diarrhea (50% ; N=2518), followed by HuCV 12% (N=587), Astrovirus 2% (N=105), Adenovirus 2% (N=102), and mixed infection about 8%. Rotavirus was detected in 48% of 7744 inpatients, 24% of 3142$B!!(Boutpatients, and 9% of 642 patients in the community. The incidence was the highest in infants aged 6-23 months (>42%), 94% of all rotavirus episodes occurred in the first 2 years of life and only 12% were detected in infants <6 months of age. The peak season of rotavirus activity was from October through March. The common rotavirus strains -- P[8]G1, P[4]G2, and P[8]G3 -- accounted for 77% of all isolates and G3 instead of G1 has been the predominant serotype since 2001. The HuCV detection rates among inpatients (N=5351), outpatients (N=1542), and patients in community (N=356) were 11%, 11%, and 16% respectively. The age distribution of HuCV diarrhea was similar to that of rotavirus. The incidence rate was the highest in infants aged 6-23 months (>21%), 91% of episodes occurred in the first 2 years of life, and 18% were in infants aged <6 months. The peak of HuCV diarrhea was from October through January, but the detection rate in this period (range: 11%-27%) was less than that of rotavirus (range: 15%-54%). The most common strain circulated was Norovirus GG$B-6(B-4 (62%), followed by GG$B-6(B-3$B!J(B16%$B!K(B, GG$B-6(B-1$B!J(B9%$B!K(B, GG$B-6(B-7$B!J(B6%$B!K(B, GG$B-5(B-2 (2%), GG$B-6(B-8 and GG$B-5(B-1 (1%), and Sapovirus SG$B-5(B-1 (2%) and SG$B-6(B-1 (1%). The age distribution and seasonality of astrovirus and adenovirus circulated in China appeared to be similar to that of rotavirus. The strain characterization of these two pathogens will be presented. Conclusion Diarrhea was second most common disease among children <5y in China and the major pathogens were viruses in inpatients. The results of epidemiology and strain characterization of these four enteric viruses provide a scientific basis to consider vaccine development against viral diarrhea in the future.