Epidermal growth factor (EGF) stimulates DNA synthesis and proliferation of thyroid cells in culture and may have an important role in the regulation of normal and neoplastic thyroid cell growth. We therefore studied paired normal and neoplastic thyroid tissue from eight patients for the presence of EGF receptors using a radioreceptor assay. 125I EGF binds to a particulate membrane fraction from both normal and neoplastic thyroid tissue with high affinity (dissociation constant ranged from 0.5 to 16.7 nmol/L). The binding is saturable, and maximal binding is achieved within 40 minutes at 37 degrees C and pH 7.5. This EGF binding is specific since it is competitively inhibited by unlabeled EGF but not by other hormones (thyrotropin, insulin, glucagon, and transferrin). The binding of EGF to thyroid neoplasms is higher than the binding to normal thyroid tissue (p less than 0.05). Thyroid tumors with a poorer prognosis appear to have higher EGF binding compared with adjacent normal thyroid tissue than have tumors with a better prognosis. EGF may have a role in the regulation of normal and neoplastic thyroid cell growth. Characterization of EGF receptors may help predict the clinical course of patients with malignant thyroid neoplasms.