This study evaluated the potential association between increased intraabdominal pressure (IAP) and abnormally low gastric intramucosal pH (pHi) (≤ 7.32) in postoperative patients and assessed its effect on patient outcome. Altogether 73 patients undergoing major abdominal surgery over a 9- month period were studied prospectively. All underwent gastric tonometry and intravesical IAP measurements three time daily. An IAP of ≤ 20 mmHg and a pHi of ≤ 7.32 were considered abnormal. The development of the following complications were also documented: hypotension [mean aortic pressure (MAP) < 80 mmHg], abdominal sepsis, renal impairment, and death. The median APACHE II score was 16 (range 5-34). Twenty-two patients bad upper gastrointestinal (GI) surgery, 27 lower GI surgery, and 24 aortic surgery; 44 of these patients underwent emergency surgery. Abnormal pHi (≤ 7.32) occurred in 36 patients while on the intensive care unit. Compared to patients with normal pHi, abnormal pHi patients were 11.3 times (3.2-43.5) [odds ratio ± 95% CI] more likely to have an increased IAP. Abnormal pHi was significantly associated with hypotension (χ2 = 6.8;p = 0.009), sepsis (χ2 = 3.7; p = 0.06), renal impairment (χ2 = 28.3; p = 0.0000001), relaparotomy (χ2 = 4.1;p = 0.04), and death (χ2 = 9.7; p = 0.002). This study demonstrated a significant clinical association between increased IAP and abnormal pHi. An abnormally low pHi was associated with poor outcome.