On December 6, 1979, three luncheon banquets were served in a New Jersey restaurant. Thirty-eight of 41 members (92.7%) of the first group became ill as did 25 of 31 members (80.6%) of the second group. None of 12 members of the third group were ill. Illness consisted primarily of diarrhea (76%), nausea, (73%), vomiting (67%), cramps (46%) and fever (18%); the median incubation period was 31 hours and median duration 24 hours. The same foods were served to all three groups, except that cole slaw was substituted for a green salad and mixed vegetables for lima beans for the third group. Consumption of green salad was associated with illness (p less than 0.0001). A total of 118 other persons who ate at the restaurant on the same day were interviewed, 60% of whom reported being ill. Green salad was significantly associated with illness for this group at lunch (p = 0.005) and dinner (p = 0.00007). Serologic studies on seven of 12 patients and on one of four exposed controls showed a fourfold or greater rise in antibody titer to Norwalk virus.