Of 92 patients with moderately severe acute pancreatitis initially studied within three weeks of onset by ultrasonic tomography, 52 developed an acute fluid collection in the lesser sac. Documentation of the ultrasound prediction of pseudocyst was achieved by surgery or autopsy in 26 cases. Spontaneous resolution of the acute pseudocyst was demonstrated by serial ultrasonography and radiogrphy in another 10 patients. Exploration exposed 3 false positive predictions of pseudocyst. Eleven other patients with a cystic configuration either refused surgery or were lost to followup. Acute pseudocyst formation is a relatively common phenomenon in the early phases of moderately severe pancreatitis. While spontaneous resolution of acute pseudocysts is frequent, in approximately 50% of cases acute pseudocysts progress to chronic pseudocysts. A distinction between acute and chronic pseudocyst is necessary since specific surgical management depends upon the phase of pseudocyst development. Unless regional sepsis supervens, acute pseudocyts of less than three weeks' duration may be followed by serial ultrasonography in the hope of spontaneous resolution. When a pseudocyst has achieved chronic status, spontaneous resolution is rare. Persistent conservative management under these conditions invites the excessive mortality and morbidity of spontaneous rupture.