[en] Objective: To study the influence of ¹³¹I treatment on the circulating sFas levels of patients with Graves' disease (GD) before and after treatment with ¹³¹I. Methods: Using the method of enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay, the levels of sFas in serum and thyroid hormone of GD 52 patients were measured before and after ¹³¹I therapy while 30 healthy subjects with age and sex matched served as controls, and the results were compared with that of the control group. Results: The sFas levels in serum of GD patients were significantly higher than that of the control group (P<0.01). The serum sFas level tended to decrease after the medication of ¹³¹I at 180 d, but still higher than that of the normal controls (P<0.05). The concentration of serum sFas was positively correlated with the titers of anti-hyrotropin (TSH) receptor antibody (r=0.595, P<0.01), but not with the other clinical parameters (FT₃, FT₄ and TSH). Conclusions: Abnormal concentration of sFas in serum can be observed in patients with GD. ¹³¹I therapy would suppress the auto-immune status in GD patients, and the circulating sFas may play a role in the pathogenesis of GD. (authors)