To determine if measurements of gastric intramucosal pH have prognostic implications regarding ICU mortality. Prospective comparison of outcome. General adult ICUs in two teaching hospitals. Eighty consecutive patients age 18 to 84 yrs (mean 63.4), 50 men and 30 women, 55% in the medical and 45% in the surgical services. Gastric intramucosal pH was measured on ICU admission and again 12 hrs later. A value of greater than or equal to 7.35 was used to differentiate between normal and low gastric intramucosal pH. Fifty-four patients had a normal gastric intramucosal pH and 26 patients had a low gastric intramucosal pH on ICU admission. The mortality rate was greater in the low gastric intramucosal pH group (65.4% vs. 43.6%; p less than .04). The frequency of sepsis and the presence of multisystem organ failure also were greater in the low gastric intramucosal pH group (p less than .01). Further stratification of patients according to gastric intramucosal pH measured 12 hrs after admission showed a greater mortality rate in patients with persistently low gastric intramucosal pH when compared with patients with normal gastric intramucosal pH during the first 12 hrs (86.7% vs. 26.8%; p less than .001). Measurements of gastric intramucosal pH on ICU admission, and again 12 hrs later, have a high specificity for predicting patient survival in this ICU patient population (77.8% to 80.6%). Furthermore, given its relative noninvasive nature, tonometrically measured gastric intramucosal pH may be a useful addition to patient monitoring in the ICU.