Two major viruses have been clearly associated with gastroenteritis: the Norwalk agent and the rotavirus. The former is associated with epidemic outbreaks in adults or schoolchildren, while the latter causes endemic illness in young children during winter. Neither can be grown in routine cell cultures. Both of these viruses, and others, can be detected in patients' stools by electron microscopy. Enzyme linked immunosorbent assay (ELISA) kits are commercially available for detection of rotavirus antigen in stools. This test, which can be readily performed in the routine laboratory without specialized equipment, offers useful diagnostic and prognostic information.