Pituitary carcinomas are only defined by their metastatic growth, which may be intracranial or systemic. To establish further morphological and immunohistochemical differences between pituitary carcinomas and adenomas, 19 ACTH-secreting adenomas (10 non invasive and 9 invasive) and 2 ACTH-secreting carcinomas with their metastases were studied for expression of the intermediate filaments keratin and vimentin and the tumor-associated antigens Ki67, proliferating cell nuclear antigen (PCNA), epidermal growth factor (EGF), cathepsin D, p53, and carcinoembryonic antigen (CEA). Immunohistochemistry was performed using avidin-biotin techniques on formalin-fixed, paraffin-embedded tissue. With the exception of one noninvasive pituitary adenoma, one carcinoma, and the metastases, all tumors contained keratin; none contained vimentin. All tumors stained negative for CEA and p53. Eleven (58.5%) adenomas and both pituitary carcinomas contained Ki67-positive nuclei; 14 (74%) adenomas and one carcinoma revealed PCNA. No correlation was found between the two markers. Seven (38%) adenomas showed a labeling index <1 % for cathepsin D, whereas none of the carcinomas or metastases did so. EGF was found in 7 (38%) adenomas and in both carcinomas. A tendency to a higher rate of EGF positivity in the invasive adenomas was observed. The metastases showed a higher labeling index, and far more intense staining results for Ki67, PCNA, and EGF than the primary tumor. The metastases also had a higher proliferation rate and growth factor content than the carcinoma itself.