Pediatric gastroenteritis places a considerable disease burden on children of the developed world. The national surveillance of gastroenteritis in Ireland is a combined virological and epidemiologic surveillance program. The objectives of this study were to characterize the norovirus (NoV) genotypes associated with viral gastroenteritis in children <or=5 y of age, and compare these strains with those detected in adult specimens. A total of five different NoV genotypes were associated with infection in Irish children [Genogroup II/type 2 (GII/2),GII/4,GII/6,GII/b,GII/14] whereas only GII/4 strains were identified in adults. This significant genotypic difference in the NoV strains associated with pediatric and adult infection was found in both community- and hospital-based infection. To assess the burden that NoV places on Irish children, the relative prevalence of norovirus, rotavirus, and adenovirus was determined in hospitalized symptomatic children <or=5 y old. Our results identified NoV as a major cause of gastroenteritis in children >or=4 mo of age and determined that NoV and adenovirus infection are equally significant in children in the first 5 y of life. This group of pediatric patients reported diarrhea as their most common symptom raising the question whether Kaplan criteria are the most effective method for clinically diagnosing outbreaks of enteric infection in pediatric patients.