Thirty-four cases of microcystic adenoma of the pancreas were studied. These benign tumors have traditionally been classified as cystadenomas, but have not been clearly distinguished from those cystic neoplasms of the pancreas that have a significant malignant potential. Microcystic adenomas are benign, usually large (mean diameter 10.8 cm), and are composed of many tiny cysts lined by small cuboidal cells containing glycogen but little or no mucin. The tumor may be found incidentally at autopsy or, more commonly, may manifest as an abdominal mass with some associated local pain or discomfort. There is no sex predilection. The patients are usually elderly, both mean and median ages being 68. A microcystic pattern is apparent both grossly and microscopically. Electron microscopy confirms the presence of intracytoplasmic glycogen and the epithelial character of the cells. Follow-up data (mean 6.4 years), available in all cases, indicate that when these tumors occur in the head of the pancreas, fatalities may result either from complications of radical surgery or from gastrointestinal or biliary obstruction. When the tumors occur in the body or tail of the pancreas, biopsy alone may be sufficient.