The authors investigated a prognostic value (with respect to the complications) of certain laboratory parameters used in the examination of the gastric juice in patients with duodenal ulcer (DU) as well as the prognostic value of certain anamnestic data. A total of 100 patients with DU who had been followed up for 5-6 years were under study. The results obtained demonstrated the most informative values for the DU-related complications turned to be the parameters of the performance of the defense mechanisms of the gastroduodenal area (the contents of mucous proteins, the coefficient of the gastric juice defense), the levels of gastric secretion and acidification (the quantity of the juice, the per hour discharge of hydrochloric acid) as well as the ratio between the coefficient of the defense and the coefficient of the pepsin aggression that reflected the ratio between the aggressive and defensive factors of the gastric juice. Defined discriminant functions permitted the prognostication of complications appearance correct within 80-90%.