The aim of the study was to investigate the role of smoking and alcohol and analgesic ingestion in the aetiology of chronic duodenal ulcer (DU). Exposure to these variables was studied during the lifetime and 1 year before the first ulcer symptom and 1 month before a diagnosis. The ulcer group of 100 patients was compared with 100 community controls matched for age, sex, and social grade. In men there was an increased risk of DU associated with smoking and alcohol ingestion both during the lifetime and 1 year before the first symptom. In women, there was an increased risk associated with smoking during the lifetime before the first symptom and with alcohol ingestion during the year before the first symptom. Analgesic ingestion was not a risk factor. In the month before diagnosis only smoking was commoner in the DU group. The data indicate that smoking and alcohol ingestion are risk factors for DU.