Pap test was used to study gastric and duodenal G and D cells, blood gastrin levels, basal and stimulated acid production, clinical manifestations, and morphological characteristics of the mucosa in 39 patients with duodenal ulcer and 13 controls. The findings enable the authors to outline a special form of peptic ulcer that is characterized by gastrin cellular hyperplasia concurrent with relative pyloric D cell deficit, gastric metaplasia in the duodenum and gastric hypersecretion. Such patients have more frequently multiple ulcers, predominantly 0(I) blood group, complication-aggravated course of the disease, and ulcer history in close relatives. Moreover, it has been demonstrated that incidence of gastric metaplasia is due to gastric hypersecretion; hyperplasia of duodenal gastrin cells is associated with incidence of gastric metaplasia in patients with peptic ulcer.