Hypothesis: Intra-abdominal hypertension exerts an effect on renal function independent of other confounding variables. Design: A prospective study of all patients admitted to an intensive care unit following abdominal surgery was undertaken between September 1, 1994, and July 31, 1997, in a single university hospital. Main Outcome Measures: Intra-abdominal pressure (IAP) was measured every 8 hours (normal IAP, 0-17 mm Hg); 18 mm Hg or higher was considered increased. Forward stepwise logistic regression determined whether intra-abdominal hypertension is an independent cause of renal impairment. Results: A total of 263 patients (174 after emergency surgery), whose mean ± SD age was 61.0 ± 18.7 years and Acute Physiology and Chronic Health Evaluation II score was 14.6 ± 7.7, were studied. Intra-abdominal pressure was increased in 107 (40.7%) of the 263 patients. Renal impairment occurred in 35 (32.7%) of the 107 patients with intra-abdominal hypertension and in 22 (14.1%) of the 156 with a normal IAP (odds ratio, 1.62-5.42). Using the Wald maximizing model, renal impairment was independently associated with 4 antecedent factors: hypotension (P=.09), sepsis (P= .006), age older than 60 years (P= .03), and increased IAP (P =.004). Conclusions: To our knowledge, for the first time in a large clinical study, IAP has been shown to be an independent cause of renal impairment, and it ranks in importance after hypotension, sepsis, and age older than 60 years. Surgeons need to be aware of the importance of intra-abdominal hypertension in patients postoperatively.