In most patients duodenal ulcer is a chronic relapsing disease. If no active maintenance treatment or eradication therapy is given after healing, around 70-100% of patients have a relapse during the first year. We conducted a double-blind multicenter study in 472 patients with duodenal ulcer. They were treated with omeprazole 20 mg every morning for four or eight weeks and when healed were randomly allocated to maintenance treatment with either omeprazole 20 mg every morning or ranitidine 150 mg at bedtime for up to six months. The patients were assessed by endoscopy at monthly intervals until healing occurred. Thereafter scheduled endoscopy was carried out after 1, 3, and 6 months of maintenance treatment or immediately in the event of a suspected relapse. Healing status (intention to treat approach) was 87% at four weeks and 93% at eight weeks. At six months the estimated remission rate was 90% for omeprazole and 82% for ranitidine (P = 0.03, 95% CI 1-15%). The incidence of adverse events was similar during the two maintenance treatments. Treatment with omeprazole 20 mg every morning maintained significantly more patients in remission than treatment with ranitidine 150 mg at bedtime.