Background: Abdominal compartment syndrome can develop within 12 hours of intensive care unit (ICU) admission in high-risk (shock/trauma, burn, pancreatitis, postabdominal aortic surgery) patients. The current standard of intra-abdominal pressure (IAP) measurement via the urinary catheter is labor intensive, and its intermittent nature could prevent timely recognition of significant changes in IAP. We propose that continuous IAP (CIAP) can be accurately measured via the irrigation port of a three-way catheter and has good agreement with the standard intermittent IAP (IIAP). Methods: CIAP was prospectively validated by comparing it with IIAP measurement in general surgical and trauma patients admitted to the ICU with a three-way urinary catheter. CIAP was measured via the irrigation port of the three-way catheter transduced to the bedside monitor as a continuous trace without intermittent clamping of the catheter. The standard IIAP measurements were performed via the urine drainage port after clamping the catheter and filling the bladder with 50 mL of 0.9% saline. Each patient had three separate paired measurements performed in standardized manner to compare CIAP with IIAP. Patients' demographics, injury severity, type of surgery, body mass index (BMI), and the paired individual IAP measurements were recorded. The paired measurements were compared using the Bland-Altman (B-A) method for comparing a new clinical measurement with an established one. Data are presented as mean ± standard error of the mean. Results: During a 6-month period (ending in July 2003), 25 patients were investigated. The mean age was 61.5 ± 4 years, 66% were men, and BMI was 29.2 ± 2 kg/m(2). Six patients had vascular surgical, four elective and three urgent general surgical interventions. There were 12 trauma patients with ISS of 23 ± 2. The CIAP was 14.2 ± 0.66 (range 2 to 24) mm Hg, and the IIAP was 14.0 ± 0.68 (range 3 to 24) mm Hg. Seventy-five percent of the measured pairs were exactly the same; in 21%, there was 1 mm Hg difference and in 4% 2 mm Hg. There was no measurement difference greater than 2 mm Hg. The mean difference between the CIAP and IIAP was 0.019 ± 0.05 mmHg. The B-A statistics revealed that the difference between the means of measurements in each individual patient was between ±1.96 SD (95% confidence intervals). The B-A scatter plot did not follow any patterns of typical systematic bias. Conclusion: CIAP measurement with a three-way urinary catheter is a simple and accurate method for monitoring IAP. It has an excellent agreement with the IIAP over wide pressure ranges and should replace the current labor-intensive intermittent technique.