This report describes an epidemiological study on health effects of participation in a triathlon and their relation to water quality. illness was reported more frequently by participants than by controls. Gastro-instestinal symptoms and head-aches were predominant ; respiratory, skin and mucosal symptoms were also reported. The risk of acquiring highly credible gastro-enteritis was significantly higher for participants (28/439) than for controls (1/217) ; odds ratio 14,7. Prolonged exposure to water of the Zegerplas tended to result in a higher risk of acquiring gastro-enteritis. These findings, together with the absence of an effect of other possible sources, led to the conclusion that the water of the Zegerplas was the most likely source of infection. Electron microscopic examination of faeces showed the presence of viruses in 6/12 participants. Because of the low number of samples and the absence of control samples, no definitive conclusions can be drawn, but viral aetiology of the gastro-intestinal symptoms is likely. Microbiological investigation showed that the faecal pollution of the Zegerplas at the time of the triathlon was considerable (thermotolerant coliforms 750/100mL ; faecal streptococci 20/100mL (geometric mean counts)) ; the source of pollution was the effluent discharge of the waste water treatment plant.