In a previous retrospective case-control study, hemoconcentration was associated with the development of pancreatic necrosis. The aim of the present study was to determine in a cohort study whether hemoconcentration is a marker for both organ failure and necrotizing pancreatitis. A cohort study was performed on patients admitted with acute pancreatitis from February 1996 to April 1997. Pancreatic necrosis was defined by findings on dynamic contrast-enhanced computed tomography scan or magnetic resonance imaging. Of 128 total patients with acute pancreatitis, 53 underwent computed tomography or magnetic resonance imaging. Eighteen of 53 had necrotizing pancreatitis. Logistic regression identified an admission hematocrit > or = 44% and a failure of admission hematocrit to decrease at 24 hours as the best binary predictors of necrotizing pancreatitis and organ failure. By 24 hours, 17 of 18 patients with necrotizing pancreatitis versus 11 of 35 with interstitial pancreatitis met one or the other criterion for necrosis (p < 0.001). By 24 hours, 13 of 15 with organ failure versus 36 of 104 without organ failure met one or the other criterion (p < 0.001). The negative predictive value by 24 hours was 96% for necrotizing pancreatitis and 97% for organ failure. Hemoconcentration with an admission hematocrit > or = 44% and/or failure of admission hematocrit to decrease at approximately 24 hours was associated with the development of necrotizing pancreatitis and organ failure. Patients who did not experience hemoconcentration were very unlikely to develop pancreatic necrosis or organ failure.