In order to determine whether the presence of obesity, defined as increased body mass index, would serve as a predictor of severity in acute pancreatitis, we have reviewed the medical records of 27 patients with severe acute pancreatitis. All patients had at least four positive Ranson's signs; all but three patients had at least five Ranson's signs. When the 13 patients with a fatal outcome were compared with the 14 who lived, neither obesity nor respiratory failure was an independent predictor of death. However, when the 27 patients were analyzed on the basis of whether they were obese (15 patients) or not obese (12 patients), obesity was an independent predictor of respiratory failure. Obesity was not a predictor of renal failure, pancreatic necrosis, or need for surgery. We suggest that obese patients with severe acute pancreatitis require close monitoring for the development of respiratory failure.