Gastric intramural pH (pHi), which has been shown to reflect the adequacy of oxygenation in peripheral tissue beds, was measured in acutely ill surgical patients in order to evaluate its value as a prognostic factor and its relation to the sepsis score. Fifty-nine surgical patients were studied on ICU admission. The stomach wall pH was calculated from the PCO2 in gastric juice and arterial bicarbonate concentration using the Henderson-Hasselbalch equation. A fall in tissue pH less than 7.32 was taken as an indication of inadequate tissue oxygenation. Patients with sepsis scores greater than 10 were considered septic. Hospital and short-term (within 72 h of admission) mortality rates were determined. A significantly higher short-term mortality rate was observed in patients having a pHi less than 7.32 (37% vs. 0%, p less than .005). Most (90%) of the septic patients had a pHi less than 7.32. The short-term mortality rate was the highest (50%) in the septic group. In this group also, a linear correlation was found between pHi and the sepsis score (r = -.43, p less than .01). Gastric pHi, however, offered no prediction for the long-term outcome.