The clinical significance of fluid collections in acute pancreatitis. Computed tomography (CT) displays fluid collections in patients with acute pancreatitis as well defined intra- or extrapancreatic homogeneous areas with low attenuation numbers. We followed, prospectively, the clinical courses of 128 patients who had CT during an episode of acute pancreatitis, to determine the natural history and clinical significance of any fluid collections that developed. Fluid collections were found in 48 (37%) of the 128 patients with pancreatitis. Thirty-eight developed in the 86 (44%) patients with alcoholic pancreatitis, five in the 33 (15%) with biliary pancreatitis and five in the nine (55%) with other causes of pancreatitis (excluding chronic pancreatitis, trauma, or malignancy). On clinical follow-up of the 48 patients with fluid collections, 21 patients required operation; two died before an operation was done and 25 did not require operation. Operations were done to treat pseudocysts (14), abscess (5), and necrosis (3). One patient had an operation for a pseudocyst and an abscess. All 14 patients requiring operation for pseudocyst had collections greater than 7 cm and 13 of these patients had alcoholic pancreatitis. Of the 25 patients not requiring operation, five were lost to follow-up, and 19 of the 20 who had spontaneous resolution of their fluid collection(s) had collections less than 6 cm in diameter. The eight patients who required operative pancreatic debridement for abscess or necrosis had multiple small fluid collections. In summary, fluid collections were found in one-third of patients with acute pancreatitis and were often associated with the development of local complications. 