Stool specimens from 156 Maryland nursing home residents, who became ill during 20 outbreaks of gastroenteritis from November 1987 through February 1988, were analyzed. All tested negative for astroviruses, enteroviruses, Group A rotaviruses, Sapporo-like caliciviruses, and enteric bacteria (i.e., Salmonella, Clostridium, and Shigella species). Eighty-two (52%) were positive for Norwalk-like viruses (NLVs), members of the family Caliciviridae. Six distinct genetic clusters within genogroups I and II of the NLVs were detected; a genogroup II (GII) virus closely related to the Camberwell virus in the NLV GII/4 genetic cluster was the predominant strain. Serologic evidence of infection with > or = 1 NLV was detected in 61 (56%) of 109 patients tested against 3 NLV antigens (i.e., Norwalk, Hawaii, and Toronto viruses). Sixteen (80%) outbreaks met the definition for an NLV outbreak. Taken together with a retrospective analysis of bacterial gastroenteritis in this same setting, these data support a major role for NLVs as etiologic agents of gastroenteritis in elderly persons.