Introduction On Wednesday, March 29, 2006, the town hall of Hermanville-sur-Mer (Calvados) notified the Departmental Directorate of Health and Social Affairs (DDASS) of a cluster of gastroenteritis cases among the sailors who had participated in a sand yachting competition held on March 25 and 26, 2006. On Thursday, March 30, the Hermanville-sur-Mer Sailing and Recreation Club confirmed the report of at least a dozen cases and described the course of the competition. Those who fell ill attributed their illness to crossing wastewater runoff from an intermunicipal wastewater treatment plant. Method A retrospective cohort study was conducted to validate and describe the outbreak and identify its source. A case was defined as any sailor who participated in the sand yachting competition and experienced diarrhea or vomiting between March 25 and 31, 2006. The risk exposures investigated were: - either exposure to wastewater runoff from the treatment plant during the first round of Pool A; - or exposure to contaminated food or drink. Data were collected via telephone calls using a list of the 77 competitors, divided into two pools. Concurrently, an environmental survey described the intermunicipal wastewater treatment system and assessed the occurrence of direct wastewater discharges onto the beach at Hermanville-sur-Mer via storm sewers on March 25 and 26, 2006. Results Seventy participants were interviewed, among whom 21 cases were identified, corresponding to an attack rate of 30%. Clinical symptoms, which mostly appeared between Sunday evening and Monday noon, included diarrhea (95.2%), vomiting (71.4%), and abdominal pain (81.0%) associated with moderate fever (61.9%). Drivers in Pool A were six times more likely to become ill than drivers in Pool B. The relative risk (RR) tended to increase with the number of laps completed during the first heat of Group A: RR = 7.00 (2.25–21.78) for drivers who completed 5 laps versus 4.80 (1.28–18.02) for drivers who completed between 2 and 4 laps. No direct discharge of untreated wastewater was detected on the beach at Hermanville-sur-Mer during the weekend of March 25–26, 2006. Conclusions The cohort study confirmed the occurrence among sand yacht pilots of an epidemic outbreak associated with crossing the treated wastewater outfalls discharged onto the foreshore of Hermanville-sur-Mer beach. The upcoming connection of the municipalities of Lion-sur-Mer, Hermanville-sur-Mer, and Colleville-Montgomery to the urban community’s wastewater treatment plant, whose outfall is located 15 km from the coastline, will eliminate the health risks associated with the current location of the wastewater outfall. Until the discharge of wastewater onto the foreshore of Hermanville-sur-Mer beach is eliminated, it is recommended that sand yacht training sessions and competitions be held away from the discharge point. (R.A.)