We investigated an outbreak of fever, most likely due to a contaminated whirlpool, among nine adults and six children residing in a summerhouse. The outbreak was characterized by a high attack rate, short incubation periods, influenza-like symptoms, and rapid recoveries, all features typical of Pontiac fever. However, the children had less-characteristic symptoms than the adults, and they did not have any sequelae. Findings on the children's chest radiographs were unremarkable, and none of the children had leukocytosis. Evidence of Legionella pneumophila infection was found in six cases: in one case by isolation of L. pneumophila serogroup 1 and detection of legionellae by PCR, and in five cases by seroconversion to the clinical isolate. Six additional cases had presumptive evidence of legionella infection, with seroconversion to Legionella micdadei antigen; a PCR assay was also positive for legionellae for one of these cases. In contrast, two adult nonusers of the whirlpool had no symptoms and no serological evidence of infection. Serological testing and cultures for other pathogens, as well as cultures of all environmental samples, were negative. This investigation demonstrates the differences between adults and children with respect to the clinical picture of Pontiac fever; furthermore, it shows that culture and PCR assay of tracheal aspirates for legionellae can be performed in a hospital setting for rapid diagnosis, although the sensitivities of these methods are low.