Ascites and pleural and pericardial effusions can be observed during acute pancreatitis. The aims of this study were to evaluate their incidence, natural history, and prognostic role in patients with acute pancreatitis. One hundred patients consecutively admitted with a diagnosis of acute pancreatitis were prospectively submitted to abdominal, pleural, and cardiac ultrasonography at admission and during follow-up. Ascites was found in 18 patients, pleural effusion in 20, and pericardial effusion in 17. Twenty-four patients of this series had severe pancreatitis; three of them died. All effusions disappeared spontaneously in patients who survived pancreatitis up to two months after dismissal. At multivariate analysis ascites and pleural effusion were demonstrated to be accurate independent predictors of severity. The respective odds ratios were 5.9 [95% confidence interval (CI), 1.5-23.0%) and 8.6 (95% CI, 2.3-32.5%). Furthermore the presence of pleural effusion, ascites, and pericardial effusion were associated with an increased incidence of pseudocyst during follow-up. Ascites and pleural and pericardial effusions are frequent during acute pancreatitis. Pleural effusion and ascites are accurate predictors of severity in these patients.