Fifty-two hepatobiliary cystadenomas and 18 hepatobiliary cystadenocarcinomas were drawn from the files of the Armed Forces Institute of Pathology and Rhode Island Hospital and studied in an attempt to correlate light microscopic features of the tumors with immunohistochemical and follow-up data. The cystadenoma patients ranged in age from 2 to 87 years at the time of initial diagnosis (mean, 45 years). All the cystadenomas were multilocular with benign cuboidal to columnar epithelium, and 44 (85%) had densely cellular spindle cell ("ovarian-like") stromata; 96% were female. Fifty-one cystadenomas were macrocystic lesions, typically lined by mucinous epithelium; one of the benign lesions was a serous cystadenoma (microcystic adenoma) reminiscent of the more commonly encountered pancreatic lesion of the same name. The cystadenocarcinoma patients ranged in age from 24 to 90 years at the time of first diagnosis (mean, 59 years); eight patients (44%) were male. All but one of the lesions were multilocular with malignant in situ (one case) or invasive tubulopapillary (15 cases), solid (one case), or adenosquamous (one case) epithelial components. Areas of preexisting benign cystadenoma were found in six (33%), an observation suggesting that benign lesions may evolve into malignant ones in some patients. Most cystadenomas and cystadenocarcinomas arose in the liver, a few in the extrahepatic biliary system (including the gallbladder). On follow-up, the cystadenoma patients in general were successfully treated by surgical excision of the lesions in toto; patients treated by subtotal resection often had persistent symptomatic disease. Four cystadenocarcinoma patients died of their tumors; another two patients were alive with persistent disease at last follow-up. In both the benign and the malignant lesions, most tumor cells were positive on immunohistochemical staining with antibodies to cytokeratin, epithelial membrane antigen, and carcinoembryonic antigen; scattered chromogranin-positive cells also appeared in a few tumors of both types. Immunohistochemistry did not yield a diagnostic immunoprofile to distinguish cystadenoma from cystadenocarcinoma or from other epithelial lesions arising within the abdominal cavity. At least two types of cystadenocarcinoma exist, one developing exclusively in female patients, usually accompanied by an "ovarian-like" stroma, which follows an indolent course; and the other, lacking the distinctive cellular stroma, seen in males, follows a more aggressive course and is more likely to result in the patient's death from tumor. It remains an open question whether the cystadenocarcinomas lacking a mesenchymal stroma, which arise in women, will follow the same aggressive course as similar lesions arising in men.