We report on the first documented outbreak of leptospirosis in Austria. In July 2010, four cases of serologically confirmed leptospirosis occurred in athletes after a triathlon held in Langau. Heavy rains preceded the triathlon (rainfall: 22 mm). The index case (Patient A) was a 41-year-old previously healthy male, who was admitted to hospital A on July 8 with a four-day history of fever up to 40°C that began 14 days after attending the triathlon event. On July 7, patient B, a 42-year-old male, was admitted to the same hospital, with signs and symptoms of kidney failure. Hemodialysis was performed every other day for 3 weeks. While the serum drawn on the day of admission was negative for antibodies against Leptospira, a specimen from July 28 tested positive with Leptospira interrogans. On July 11, patient C, a 40-year-old male, was admitted to hospital B for nephritis. On July 14, patient D, a 44-year-old male, was admitted to hospital C with a ten days history of intermittent fever, mild dry cough and headache. Our report underlines that in Austria recreational users of bodies of freshwater must be aware of an existing risk of contracting leptospirosis, particularly after heavy rains. The suppressive influence of a triathlon on the immune system is well documented and therefore an outbreak in this population group can be seen as a sensitive indicator concerning possible risk for the general population.