Objectives: To assess the potential role of consumption of shellfish (particularly raw oysters) and tap water in the winter epidemic of acute diarrhoea in France. Design: Population based, case-control study during the 1995–6 winter epidemic of acute diarrhoea in France. Setting: A national network comprising 1% of general practitioners in France. Subjects: 568 pairs of cases and controls consulting in general practice and interviewed by 209 doctors from 26 December 1995 to 31 January 1996. Cases and controls were matched for age, doctor, and time of consultation. Main outcome measures: Adjusted relative risk of diarrhoea estimated from conditional logistic regression. Results: The risk of acute diarrhoea was not increased in people who had recently eaten raw oysters (odds ratio 1.1; 95% confidence interval 0.9% to 1.4%) or other shellfish such as clams, cockles, and mussels, or in those people who usually consumed tap rather than bottled water (0.8; 0.6% to 1.1%). The risk was, however, increased in people who had had recent contact with a person with diarrhoea, either within the household (adjusted odds ratio 5.0) or in the workplace (3.1), and in people who lived with a child ≤2 years of age (1.6). Recent treatment with either oral penicillin or cephalosporin was also independently associated with acute diarrhoea in winter. Conclusions: The winter epidemic of acute diarrhoea in France is probably not caused by consumption of either shellfish or tap water. A viral aetiology, however, is suggested by the speed with which the acute diarrhoea is transmitted. Acute diarrhoea has a strong seasonal pattern in France with a large outbreak starting regularly at the beginning of January and lasting about four to six weeks The results of this case-control study ruled out the consumption of shellfish and tap water as causes of the winter outbreak of acute diarrhoea in France ß-Lactam antibiotics, including penicillins and cephalosporins, were associated with the outbreak The speed with which acute diarrhoea is transmitted is compatible with a viral aetiology