The aim of this study was to gain experience concerning efficacy and safety of famotidine, the new H2-receptor antagonist, for the maintenance of duodenal ulcer disease. 344 patients whose acute duodenal ulceration had recently been healed under famotidine or ranitidine were recruited for a year maintenance treatment with 20 mg bedtime dose of famotidine or placebo. 167 patients were treated with famotidine over 6 and 52 over 12 months. The corresponding numbers in the placebo control were 177 and 21. A life table method of analyses showed that the ulcer relapse rate was consistently and significantly (p less than 0.01) lower on famotidine than on placebo after 6 months (26% [43/167] versus 55% [98/177]). Of the 52 patients treated with 20 mg famotidine at night for further 6 months 7 (14%) developed an ulcer relapse. Of the 21 patients treated for further 6 months with placebo 5 (24%) showed an acute ulcer crater at endoscopy. Famotidine was well tolerated in the longterm therapy. The results confirm the efficacy and safety of famotidine in the prevention of duodenal ulcer relapse for at least 6 months.