Nine members of a party of 24 people who attended a birthday party fell ill with gastroenteritis between one and three days later. A cohort study undertaken using a postal questionnaire showed that illness was associated with having eaten raw oysters. Six of the cases had their stools examined and two were positive for small round structured virus. The illness was brief and none of the cases had consulted a general practitioner. Had the cases not been part of a party they would not have been identified. The oysters were grown in English coastal waters in grade B oyster beds. They underwent depuration treatment before they were sold for consumption. More work is needed to protect oyster beds from contamination and to identify methods to render oysters safe for consumption.