The aim of this study was to investigate the association between H 2 -receptor antagonists and acute pancreatitis. The automated database of the Medicines Monitoring Unit (MEMO) was used to carry out a case-control study, supplemented with information on possible confounding factors from hospital and GP medical records. Cases were patients hospitalized with a computerized diagnosis of acute pancreatitis, and two sets of controls were drawn from (1) the study population and from (2) the same GP practice as the case. Current or 60-day exposure to cimetidine and ranitidine was analysed. In adjusted analyses, cimetidine exposure and ranitidine exposure were associated with an increased risk of hospitalization for acute pancreatitis, as were alcohol abuse and cholelithiasis. The risks were lower in unadjusted analyses, suggesting that the association is confounded, although they did not disappear completely. A possible explanation is that data on confounding were incomplete. This study cannot discount the existence of an association between H 2 -antagonists and acute pancreatitis, and highlights the diculties involved in obtaining complete and accurate data on confounding factors that are not collected routinely.