Ultrasound has proven invaluable in detecting and evaluating pancreatic pseudocysts, and it is now a standard test to rule out complications of pancreatitis. In reviewing the authors' experience with 122 patients treated surgically for a pancreatic pseudocyst, five patients were identified in whom an ultrasound demonstrated a pseudocyst that was associated with an unexpected cancer at the time of operation. A sixth patient, with a pseudocyst documented by ultrasound, died prior to surgery and was found at autopsy to have metastatic common bile duct carcinoma. There was little difference in presenting symptoms, age, frequency of alcoholism, or physical findings compared with patients with pseudocysts secondary to pancreatitis. In two patients, pseudocysts were found in the tail of the pancreas at operation, in addition to carcinoma. In the other three patients, no pseudocyst was found; however, a subcapsular splenic hematoma was present in one. Five patients had metastatic disease, three from pancreatic adenocarcinoma, one from islet cell carcinoma, and one from a common bile duct carcinoma. One patient with a pancreatic adenocarcinoma confined to the head underwent a Whipple procedure and has no evidence of disease 6 months later. Malignancy may cause or coexist with pancreatic pseudocysts. Ultrasound is often not helpful in distinguishing pseudocysts associated with malignancy from those associated with pancreatitis. Biopsy should be performed to rule out malignancy when operating for pancreatic pseudocysts.