The authors have performed an investigation of the association between ulcer disease of the duodenum and bleedings from the ulcers in 204 patients after transplantations of the kidney with the antigen of HLA system histocompatibility. Duodenal ulcers were found in 25 patients (12.9%), in 11 of them the ulcers were complicated by bleedings which was 5.4% of the total number of the patients with the transplanted kidney. The evaluation of the HLA antigen distribution has revealed an association of duodenal ulcers with A2 and B13 antigens which took place significantly more often than in a group of practically healthy people (RR = 1.85 and 1.9 correspondingly). In patients with the transplanted kidney and following ulcerous bleedings this association was more obvious (RR = 2.32 and 3.74 correspondingly). According to the calculated etiological fraction 40% of the patients can be supposed to have the etiological association of the disease with A2 antigen and 20% of the patients with B13 antigen. A typical feature was the presence of possible "protective" A9 antigen which was found in the group under study two times more seldom than in the control group (RR = 0.25). A comparison of the distribution of HLA antigens in the patients in whom there developed bleeding duodenal ulcers with those who had no such complications has also established the association of the disease with HLA A2 and B13 antigens. The probable "protective" A9 antigen was also found. Since the kidney transplantation is preceded by determination of the recipient's HLA phenotype, these data can be used as prognostic criteria as well as the indications to performing preventive antiulcerous therapy in patients with high risk of the development of ulcerous lesions of the gastrointestinal tract.