An outbreak of acute nonbacterial gastroenteritis occurred among residents and staff in a nursing home in Baltimore, Maryland, in December 1980. A total of 101 residents and 69 staff members were surveyed by questionnaire. The attack rate (defined as acute onset of vomiting or two or more loose stools per 24 hours) was 46% in each group. Illness was brief and mild; no patients were hospitalized, and there were no deaths. Person-to-person transmission was documented by temporal clustering of cases (the demonstration of higher rate of illness among residents exposed to an ill roommate one or two days earlier than among those not similarly exposed; relative risk = 3.74), by a higher rate of illness among employees having daily contact with residents than among those without such contact (57% vs. 17%, p less than 0.01), and by secondary transmission to household contacts of ill employees (secondary attack rate = 33%). Three of 11 serum pairs from patients demonstrated a fourfold increase in antibody titer to the Norwalk virus between acute- and convalescent-phase specimens. The analysis of temporal clustering of cases was particularly useful in documenting person-to-person transmission in this outbreak and might be used for this purpose in other outbreaks caused by Norwalk or Norwalk-like viruses, as well as in outbreaks associated with other infectious organisms.