The epidermal growth factor receptor (EGFr) family has been increasingly recognized as an important component in the control of normal cell proliferation and the pathogenesis of cancer. We have studied EGFr expression in 104 cases of hyperparathyroidism by immunocytochemistry (ICC) and by in situ hybridization (ISH). Using two different monoclonal antibodies, ICC for EGFr was performed on 66 cryostat sections and 38 wax-embedded parathyroid glands. ISH was performed on 49 of these glands using a cocktail of three anti-sense probes to EGFr mRNA and a nonspecific control probe to human HPV-18 virus. Breast and prostate tumors were employed as positive controls for both ICC and ISH. Controls demonstrated positive EGFr staining. None of the 104 parathyroid glands showed any ICC positivity. ISH displayed positive staining for EGFr mRNA in five of six carcinomas and eight of nine nonrenal hyperplastic glands. Only 3 of 15 adenomas and 3 of 19 renal hyperplastic glands showed positive staining. This difference was statistically significant between adenoma and carcinoma (p < 0.05) and between adenoma and nonrenal hyperplasia (p < 0.01) (Tukey's multiple comparison test). This study demonstrates that EGFr mRNA is present in parathyroid tumors. Expression in carcinoma and nonrenal hyperplasia is significantly different from adenoma and renal hyperplastic glands. In contrast, ICC failed to demonstrate EGFr protein expression. These findings suggest that either receptor numbers are too low to detect by ICC or there is a failure of mRNA translation. More studies are needed to establish whether the EGFr plays a role in the development of parathyroid cancer or hyperplasia.