This case-control study investigated a cluster of respiratory illness among patrons of a restaurant. Of 173 patrons interviewed, 117 (68%) were ill. Symptoms included myalgias (93%), headache (87%), and fatigue (79%). The mean incubation period was 49 h and the mean duration of illness was 71 h. Patrons aged >15 years were more likely to have been ill than younger patrons (odds ratio [OR], 2.96; P=.002); 58% of persons who were ill sat near a large fountain, compared with 18% of respondents who were not ill (OR, 7.5; P=.005). Legionella anisa was cultured from water samples obtained from the fountain pool. Of 22 individuals who were ill, 11 (50%) had a > or =4-fold increase in the titer of antibody to that strain of L. anisa from acute-phase to convalescent-phase serum samples; 3 others (14%) had persistently elevated titers of > or =512; of a group of 20 individuals who had not been exposed to the restaurant, none had titers of >128. Pontiac fever should be considered as a diagnosis during acute outbreaks of influenza-like illness with a high attack rate and no other identified etiology.