On 6 November 2001, a possible case of leptospirosis was notified in a canoeist following a white-water event on the River Liffey. It emerged that a second race participant was also a possible case. An outbreak control team coordinated the epidemiological investigation, laboratory investigation, environmental assessment, communication and control measures. A cluster of six laboratory-confirmed cases of leptospirosis, serologically Leptospira interrogans serogroup Icterohaemorrhagiae was found. The attack rate was 9·2% (6/65). Fever, chills, red eyes and shortness of breath were significantly associated with being a confirmed case. Five cases were hospitalized. Swallowing more than one mouthful of water was associated with an increased risk of developing leptospirosis. Increased rainfall and release of hydroelectric water may have contributed to this outbreak. A multidisciplinary approach and use of the Internet and e-mail facilitated rapid and effective communication.