OBJECTIVE To investigate gastric and duodenal mucosal blood flow (MBF) in different phases of gastric ulcer (GU) and duodenal ulcer (DU) and its relation both to Helicobacter pylori (HP) infection and mucosal disorders. METHODS Upper endoscopy and histological examinations (score of inflammation, atrophy, metaplasy) were performed in 407 patients with DU and 103 with GU. Gastric and duodenal MBF were assessed by the hydrogen gas clearance technique in 102 DU and 95 GU patients. HP was detected by histology. Gastric secretion was measured in the interdigestive period and after stimulation by pentagastrin. RESULTS Lowering of MBF in gastric antrum and duodenum was observed in DU and GU patients only with score 3 of HP infection. DU healing is accompanied with a decrease of HP value and improvement of mucosal histology. At the same time MBF exhibits a significant rise: in the duodenum (by 45%) at the stage of white scar; in gastric antrum (by 26%) and body (by 40%) at healing stage, but a decrease in white scar. During healing of GU gastric MBF reached maximum in active ulcer but in white scar MBF was significantly lower. MBF at ulcer margin and MBF in ulcer crater was the same (30 ml/min/100 g) with MBF in the region of white scar with enhanced inflammation (score 2.1) before GU relapse. CONCLUSIONS Changes of MBF in different phases of ulcer are, in part, determined both by HP and by mucosal morphological disorders. The ratio MBF increase in ulcer healing/MBF reduction in ulcer relapse is the same (30% from optimal) and it is restitution entity. The MBF level of 30 ml/min/100 g was assessed as crucial in ulcerogenesis. Lowering MBF in mucosa with remaining inflammation in the scar region may predict GU relapse.