In November 1999, 93 cases of Legionnaires' disease were identified during an outbreak linked to a trade show in Kapellen, Belgium (43 confirmed cases, 12 probable cases, and 38 possible/clinical cases). Five patients died. The epidemiological investigation revealed an association between time spent at the fair, exposure to certain areas of the tent, and the onset of the disease. PCR (polymerase chain reaction) analyses showed that a water feature and a fountain were contaminated with Legionella. This article describes the outbreak, the results of the investigation into the source of contamination, and the control measures taken.