Oncogenesis of anaplastic carcinoma of the pancreas is a subject of controversy, because it shows sarcomatous nature with extremely poor prognosis. We herein report an unusual case of anaplastic carcinoma occurring with a recurrent mucinous cystic neoplasm in a 38-year-old female. A 10-cm retroperitoneal cystic mass was pointed out in the first pregnancy and a probable diagnosis of mucinous cystic neoplasm was made in October 2000. She refused surgery first and delivered her baby uneventfully. During her second pregnancy in 2002, however, she presented hematemesis and underwent urgent distal pancreatectomy, splenectomy and partial resection of the gastric wall where the tumor perforated. A diagnosis of borderline-type mucinous cystic neoplasm with ovarian-like stroma was made. Nine months later, CT visualized a recurrent cystic tumor near the pancreatic stump, which was subsequently resected. Pathology revealed that the tumor was composed of two different components of borderline-type mucinous cystic neoplasm and anaplastic carcinoma. The latter was intensely positive for vimentin, CD68, p53 and focally for cytokeratin, suggesting both sarcomatous and carcinomatous differentiation. She survived four years after the second surgery without tumor recurrence. Although the origin of anaplastic carcinoma has not been determined yet, it should be remembered that anaplastic carcinoma can occur in association with mucinous cystic neoplasm of more benign histology. Oncogenesis of anaplastic carcinoma of the pancreas is a subject of controversy, because it shows sarcomatous nature with extremely poor prognosis. We herein report an unusual case of anaplastic carcinoma occurring with a recurrent mucinous cystic neoplasm in a 38-year-old female. A 10 -cm retroperitoneal cystic mass was pointed out in the first pregnancy and a probable diagnosis of mucinous cystic neoplasm was made in October 2000. She refused surgery first and delivered her baby uneventfully. During her second pregnancy in 2002, however, she presented hematemesis and underwent urgent distal pancreatectomy, splenectomy and partial resection of the gastric wall where the tumor perforated. A diagnosis of borderline-type mucinous cystic neoplasm with ovarian-like stroma was made. Nine months later, CT visualized a recurrent cystic tumor near the pancreatic stump, which was subsequently resected. Pathology revealed that the tumor was composed of two different components of borderline-type The latter was intensely positive for vimentin, CD68, p53 and focally for cytokeratin, suggesting both sarcomatous and carcinomatous differentiation. Although the origin of anaplastic carcinoma has not been determined yet, it should be remembered that anaplastic carcinoma can occur in association with mucinous cystic neoplasm of more benign histology.