Microcystic adenoma (serous cystadenoma) of the pancreas (MA) is an unusual benign tumor of uncertain histogenesis. We have studied 14 cases of MA from 11 women and three men. The average age at diagnosis was 64 years. Six tumors were discovered incidentally. Tumors varied from 2.5 to 12 cm in greatest dimension and all were multicystic; eight tumors were located in the pancreatic head, two in the body, and three in the tail. Each tumor was composed of variably sized cysts lined by simple cuboidal or flattened, focally glycogen-rich epithelium. The stroma was variably collagenized and showed highly vascularized, delicate to broad fibrous septae, which focally contained dystrophic calcification, cholesterol clefts, and hemosiderin. Immunohistochemical studies were performed on eight cases to determine the cell of origin. Epithelial membrane antigen and a low-molecular-weight keratin, detected by monoclonal antibodies PKK1 or AE1/AE3, were diffusely seen in tumor cells of all cases. Tumor cells were uniformly negative for carcinoembryonic antigen, chromogranin, insulin, glucagon, somatostatin, vasoactive intestinal peptide, pancreatic polypeptide, and a low-molecular-weight keratin detected by monoclonal antibody PKK2. Tumor cell antigen reactivity most resembled that seen in normal centroacinar and ductal cells. Electron microscopy of seven cases showed primitive tumor cells with irregularly spaced, short, blunt microvilli, luminal occluding junctions and belt desmosomes, bundles of filaments including dense bodies in both apical and basal cell cytoplasm, sparse organelles, and variable but often pronounced amounts of glycogen. These ultrastructural features most closely resembled the normal pancreatic centroacinar cell. Based on both immunohistochemical and ultrastructural features described above, we conclude that the centroacinar cell is the cell of origin of MA.