Cystic lesions and neoplasms of the pancreas are uncommon, but they are of special interest because they can usually be cured by resection. During the last decade, the spectrum of these tumors has increased considerably. We present a series of five cystic lesions of the pancreas that differ from all categories described so far. The patients affected by these tumors were three men and two women (mean age, 57 y). Four lesions were unifocal and involved the head of the pancreas; one was multifocal and involved the pancreatic head and tail. Grossly, these tumors presented as unilocular or multilocular thin-walled cysts that contained turbid fluid, or, in two cases, blood, and lacked any communication with the duct system. Microscopically, the cysts were lined by cuboidal to columnar mucin-producing cells, supported by a small band of dense fibrous stroma. Immunocytochemically, the epithelial cells were positive for cytokeratins 7, 8, 18, 19, and 20 (except one), and Ca 19-9 but were negative for trypsin, CEA, synaptophysin, chromogranin A, calretinin, and alpha-inhibin. In four of the five lesions, the epithelial cells expressed MUC5AC, and in one of the five, MUC1. MUC2 and MUC6 were not expressed in any of the lesions. The stromal cells lacked the nuclear progesterone positivity that is typical of mucinous cystic neoplasms. During a mean follow-up period of 2 years, there were no recurrences or cases of malignant transformation after resection. The results suggest that these cystic lesions are distinct from mucinous cystic neoplasms, the most important entity in the differential diagnosis. Because they may represent a nonneoplastic cystic change of the pancreas, we propose the descriptive term mucinous nonneoplastic cyst for these tumors of unknown pathogenesis.