Epidermal growth factor (EGF) has been localized in several human neoplasms and has been shown to have a significant correlation to prognosis. We investigated the potential role of the EGF receptor (EGF-R) system in pituitary tumorigenesis by examining the expression of EGF and EGF-R in the different types of human pituitary adenomas. EGF was identified by immunohistochemistry in all cell types of the nontumorous adenohypophysis and in all types of morphologically characterized functional (n = 9) and nonfunctional (n = 17) adenomas. To confirm local EGF synthesis, ribonucleic acid (RNA) from human pituitary adenomas was reverse transcribed and PCR amplified. Transcript signals of the expected size were identified, with marked variation in 41 of 48 adenomas. To assess possible secretion in vitro, EGF was measured in pituitary tumor culture medium. No measurable quantities of EGF were present in conditioned culture medium from all 35 adenomas examined, consistent with the rapid uptake of EGF by unoccupied functional EGF-R sites. Using a specific monoclonal antibody that recognizes the extracellular ligand-binding domain of the human EGF-R, we found EGF-R in cells in the normal pituitary and in some functional and nonfunctional adenomas with extremely variable intensity. By RT-PCR, EGF-R messenger RNA (mRNA) expression was also identified, with marked variation in all 48 adenomas examined and in the nontumorous pituitary. The highest degrees of EGF-R mRNA expression were present in somatotroph adenomas and the aggressive silent subtype 3 adenomas. Tumors from patients with recurrent acromegaly demonstrated significantly higher levels of EGF-R mRNA than those from patients with nonrecurrent disease. In conclusion, EGF and EGF-R are expressed in a variable manner in all types of human pituitary adenomas. The overexpression of EGF-R in recurrent somatotroph adenomas and aggressive silent subtype 3 adenomas suggests a selective mechanism for the EGF/EGF-R family in the growth of aggressive pituitary tumors.