A multicentre maintenance trial was conducted in 63 peptic ulcer patients to investigate the efficacy and safety of continuous cimetidine maintenance therapy for up to 5 years. In Amsterdam the male patients were investigated yearly for possible endocrinologic changes and to monitor gastric acid secretory capacity. Patients with healed ulcers entered maintenance treatment with a 400-mg bedtime dose of cimetidine. The yearly relapse rates decreased from 20% to 6% from year 1 to year 4. Adverse reactions observed in this study were not different from those previously reported with cimetidine. Endocrinologic studies in 10 male patients showed a small but consistent increase in basal gastrin in the 3rd and 4th year, stabilizing in the 5th year. Prolactin levels showed a gradual increase during the first 3 years but remained well within the normal range. In the 4th and 5th year they decreased again; however, the changes noted were rather modest. It is concluded from this study that weak nocturnal acid inhibition with cimetidine maintenance treatment not only continues to be effective and safe beyond 1 year but also reduces the risk of relapse over time. No relevant endocrinologic abnormalities are to be expected with this regimen of modest nocturnal acid suppression.