Serum levels of epidermal growth factor (EGF) were investigated in 31 patients with differentiated carcinoma of thyroid. Patients with carcinoma had significantly decreased basal levels of serum EGF, however this decrease in serum EGF occurred only in the group of patients which had been evaluated at an interval of six weeks after ablation of residues of normal thyroid with radioiodine. In patients that had been treated with radioiodine for cancer this decrease was not observed. The change in serum EGF was not dependent on thyroidal function and did not correlate with the serum thyreoglobulin level. It is presumed that decreased serum EGF is a consequence of functional changes of platelets after whole body irradiation at ablative dose of radioiodine.