Several authors have demonstrated the presence of estrogen receptors (ER) and progesterone receptors (PR) in thyroid tissue, generally using dextran coated charcoal method (DCCA). The aim of the study was to measure ER and PR in thyroid specimens using an immunoenzymatic method, and to evaluate the meaning of different prevalence of ER and PR in malignant and benign thyroid disease, as compared with normal thyroid tissue. We have measured ER and PR in thyroid tissue from 28 benign and 20 malignant thyroid lesions, and in 38 samples of surrounding normal thyroid tissues. The sensitivity of ER-EIA and PR-EIA was 1.0 and 1.5 fmol/mg protein, respectively. In thyroid carcinoma the frequency of ER positivity (ER+) was 7/20 (35%); it was significantly higher in the surrounding normal tissue (15/20; 71%) (p = 0.03). In benign thyroid disease, the prevalence of ER+ was 11/28 (39%) and in the surrounding normal tissue it was 11/18 (61%) (p = not significant). PR+ was detected in 7/20 (35%) thyroid cancers and in 15/28 (53%) benign lesions without significant difference with the frequency detected in the surrounding normal tissues. ER and PR concentrations (mean +/- SD) in thyroid cancer was 2.2 +/- 2.2 and 2.2 +/- 2.9 respectively, similarly to that detected in benign thyroid disease and in normal tissue. The simultaneous presence of ER and PR (ER+PR+) was also evaluated. We have found that the frequency of ER+ PR+ was significantly higher in benign lesions (8/28; 28.6%) as compared with malignant samples (1/20; 5%) (p < 0.05); the frequency of ER+PR+ was significantly higher in normal tissue surrounding the malignant lesions (9/20; 45) (p = 0.003). Our data indicate i) EIA method is appropriate to detect ER and PR in thyroid tissue. ii) The frequency of ER+ and ER+PR+ specimens is significantly higher in normal thyroid tissue than in pathologic tissues. This indicates that ER and PR immunoassays may be useful tools to evaluate the normal biological activity of thyroid cells.