The spectrum of gastric stress ulceration includes early erosive gastritis, which develops within 48 h of injury, and a late ulcerative component. Previous clinical studies with young healthy volunteers as controls implicated increased back diffusion of hydrogen ions in the pathogenesis of stress ulceration. The gastric mucosal integrity of injured patients is compared to an age-matched control population of patients who had undergone intra-abdominal operations. These control patients suffered operative injury similar to that of the injured patients and yet, unlike injured patients, they did not develop clinically overt stress ulceration. The patients were studied during the 1st 2 days following injury and/or operation. No differences were found in the gastric mucosal permeability to hydrogen ions, as measured by back diffusion of hydrogen ions (H+BD) and by absorption of lithium (.DELTA.(Li/PSP [phenosulfonphalein]), between the injured patients and their age-matched controls, despite the subsequent development of clinically overt stress ulceration in 25% of the injured patients. Likewise, there were no differences between the traumatized patients who developed stress ulceration and either their age-matched controls or the remainder of the traumatized patients. Increased back diffusion of hydrogen ions may not be implicated in the erosive gastritis which is the early component of stress ulceration.