Development of stress ulceration and the potential for bleeding and haematemesis that ensues is an important complication encountered in critically ill patients. Many theories have been postulated for the aetiology and pathophysiology of the ulcers. The aim of this study was to study stress gastritis and the role played by ACE gene polymorphism in critically ill patients at risk for the development of stress ulceration. Forty patients from the admissions of the Critical Care Centre, Cairo University and thirty controls from the healthy adult population were subjected to the study. For the study group, APACHE II scores were calculated on the first and third days of admission, upper GI endoscopy was done after the third day and the patients ACE gene I/D polymorphism was determined. The results showed a significant difference in the ACE gene polymorphism between the two groups, with 97.5% of the study group showing DD genotype vs. 57.6% in the controls (p<0.01), indicating a link between the DD genotype and critical illness. 32 patients (80%) showed evidence of stress ulceration, of whom 8 patients (20%) had severe gastritis. 18 patients (45%) developed haematemesis, of whom 6 patients (15%) became haemodynamically unstable as a result. There was significant relationship between APACHE II score on admission and a deteriorating APACHE II score over the 1st 3 days and the development of stress ulceration. Statistical analysis showed no relationship between the risk factors, total number of risk factors present in individual patients; prophylaxis medication used and stress ulceration or haematemesis.