Background - On March 29, 2006, the Calvados Departmental Directorate of Health and Social Affairs was alerted to a cluster of gastroenteritis cases among sailors who had participated in a sand yachting competition on March 25 and 26. Investigations were conducted to confirm the outbreak, identify its cause, and propose control and prevention measures. Method - A retrospective cohort study of the sailors was conducted via telephone survey. The operation of the wastewater treatment facilities near the beach was described. Results - Twenty-one cases (18 men, 3 women, median age: 20 years) were identified among the 77 participants. Clinical characteristics were consistent with a viral etiology. Participation in one of the competition’s heats, whose course crossed treated wastewater discharged onto the beach foreshore, was associated with the onset of gastroenteritis (RR=6.35 [2.05–19.64]). The relative risk increased with the number of laps completed. No discharge of raw wastewater was detected. Discussion - Recommendations - This episode, attributable to the ingestion of wastewater spray, raises the question of the vulnerability of sand yacht pilots to coastal discharges, which should be identified and characterized. (R.A.)