To determine the incidence and the natural history of retroperitoneal masses complicating acute pancreatitis, 104 cases of acute alcoholic pancreatitis were evaluated prospectively for mass formation. Abdominal masses detected by physical examination and serial x-ray films of the upper portion of the gastrointestinal tract were localized to the retroperitoneum by additional contrast studies, including abdominal angiography. Nonoperative management was urged only for patients with an asymptomatic mass. An abdominal mass developed in 19 patients (18%). In eight of these, it disappeared rapidly, but in 11 (11%), it persisted, and was considered to be a pancreatic pseudocyst. Eight of the 11 patients were treated nonoperatively, and the mass resolved without complication three weeks to three months after diagnosis. In three patients, a pseudocyst was confirmed at laparotomy. Exploration was justified by an unstable clinical course in only one instance. A routine surgical approach to an asymptomatic retroperitoneal mass developing after acute alcoholic pancreatitis may not be necessaary in patients who are improving clinically because the mass may resolve without complication.