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Elevated intra-abdominal pressure (IAP) is a frequent cause of morbidity and mortality among the critically ill. IAP is most commonly measured using the intravesicular or “bladder” technique. The impact of changes in body position on the accuracy of IAP measurements, such as head of bed elevation to reduce the risk of ventilator-associated pneumonia, remains unclear.Prospective, cohort study.Twelve international intensive care units.One hundred thirty-two critically ill medical and surgical patients at risk for intra-abdominal hypertension and abdominal compartment syndrome.Triplicate intravesicular pressure measurements were performed at least 4 hours apart with the patient in the supine, 15°, and 30° head of bed elevated positions. The zero reference point was the mid-axillary line at the iliac crest.Mean IAP values at each head of bed position were significantly different (p < 0.0001). The bias between IAPsupine and IAP15° was 1.5 mm Hg (1.3–1.7). The bias between IAPsupine and IAP30° was 3.7 mm Hg (3.4–4.0).Head of bed elevation results in clinically significant increases in measured IAP. Consistent body positioning from one IAP measurement to the next is necessary to allow consistent trending of IAP for accurate clinical decision making. Studies that involve IAP measurements should describe the patient’s body position so that these values may be properly interpreted.