Ranitidine, given twice daily, increases the rate of healing of duodenal and gastric ulcers, irrespective of geographical location. The majority of ulcers (90-100%) heal after 2 months of treatment. Ulcers resistant to ranitidine treatment are uncommon and ranitidine heals ulcers resistant to other drugs, e.g. cimetidine. Healing of ulcers has been accompanied by an insignificant number of mild and reversible side effects. However, about 80% of healed ulcers recur within 1-2 years of stopping treatment. Continued treatment, with nocturnal administration of one half the healing dose of ranitidine, reduces the rate of recurrence during the first 12 months after healing to about 30% for duodenal ulcers and 20% for gastric ulcers. There is considerable variation within and between different countries. The recurrent ulcers seem to be clinically less severe than recurrences without maintenance treatment and are easily rehealed. If remission is maintained for more than 12 months during continued therapy, recurrence is uncommon. No significant adverse effects have been observed during maintenance treatment lasting nearly 3 years. The mechanisms whereby ranitidine heals ulcers and maintains remission are undefined but some possible processes are discussed.