A total of 61 patients with frequently relapsing duodenal ulcer were examined. Of these, 18 patients were in the acute phase and 43 experienced remission. Using 1 mm2 of the mucosa, measurements were made of the counts of duodenal and pyloric G-cells (by immunomorphologic assay), of the absolute and relative counts of T and B lymphocytes, the content of IgA, IgM and IgG, histamine and serotonin (by fluorometry) in the blood, and of the concentration of uropepsin in the urine. In the stages of exacerbation and remission, the patients suffering from duodenal ulcer with hyperplasia of G-cells manifested, as compared with the analogous patients without hyperplasia, a decrease of the absolute and relative counts of T cells, especially of those of B cells, combined with a rise of the content of IgM and IgG during exacerbation, followed by its returning to normal in the phase of remission. Over one year part of the duodenal ulcer patients with hyperplasia of G-cells received preventive treatment with ranitidine, which resulted in a tendency towards the lowering of the count of pyloric G-cells and the rise of the absolute and relative counts of T cells.