Four cases involving cystic endocrine neoplasms of the pancreas and liver are reported. Because of their rich collateral blood supply, islet cell tumors of the pancreas, even if large in size, rarely undergo central or cystic degeneration. However, failure to appreciate that a small percentage of these neoplasms may mimic benign pancreatic pseudocysts by their clinical and radiological appearance can lead to inappropriate surgical therapy. Ultrasound, computerized tomography, and/or angiography are rarely helpful in distinguishing between benign and neoplastic cysts. The definitive diagnosis can be made with assurance only by obtaining a generous biopsy of the cyst wall or any intracystic excrescences for histologic examination. Functional cystic tumors of the pancreas or liver should be excised totally whenever possible, and efforts should be made to remove as much of the tumor mass as possible even when a curative resection cannot be accomplished. Internal drainage may be acceptable as palliation for large, unresectable tumors.