Three patients with pancreatic mucinous cystic neoplasms having a sarcomatous stroma are reported. The tumors occurred in two women and one man, aged 48, 66, and 67 years, respectively. Symptoms included epigastric pain or a palpable abdominal mass or both. Radiographically, all the tumors were large, multicystic, and located in the tail of the pancreas. Histologically, the cystic component was lined by mucin-producing, columnar to cuboidal epithelium composed of benign to atypical to overtly malignant cells (cystadenocarcinoma). Immunohistochemistry showed the epithelial component of all three tumors to be positive with cytokeratin, epithelial membrane antigen, carcinoembryonic antigen, and the pancreatic epithelial mucin antibodies CA 19-9 and DUPAN 2. In all three cases, an undifferentiated malignant spindle cell (sarcomatous) stroma was intimately associated with the epithelial component. The sarcomatous component showed variable reactivity with vimentin (all three cases), muscle-specific and smooth-muscle actin (all three cases), S-100 protein (1/3), Leu-7 (one case), and estrogen and progesterone receptors (in two cases). Two patients died with widespread abdominal disease within 15 months of diagnosis, one of whom had an omental metastasis entirely composed of the sarcomatous stroma. The third patient was alive and free of disease at 16 months after diagnosis. Our findings document the existence of a sarcomatous component in pancreatic mucinous cystic neoplasms; this component appears to be responsible for the highly malignant behavior of the tumors.