The Fas/Fas ligand system induces apoptosis, while soluble Fas (sFas) blocks the system and soluble Fas ligand (sFasL) functions to induce apoptosis. The assay of nuclear matrix protein (NMP) released from dead or dying cells can be used to quantitate cell death. Therefore, we evaluated the relationship among serum levels of NMP, sFas, and sFasL in patients with Graves' disease. We measured serum levels of sFas, sFasL, NMP, thyroid hormones and TSH receptor antibody in 20 normal control subjects (5 men, 15 women; mean age, 44.3 years), 32 patients with untreated Graves' disease (4 men, 28 women; mean age, 44.1 years), and 10 patients with Graves' disease treated by methimazole (3 men, 7 women; mean age 39.2 years). Serum NMP was significantly lower (10.4 +/- 4.3 IU/ml, p < 0.02) in patients with untreated Graves' disease than in patients with treated Graves' disease (16.4 +/- 7.3 IU/ml) and control subjects (15.3 +/- 8.9 IU/ml). Serum sFas and sFasL were significantly higher in patients with untreated Graves' disease than in patients with treated Graves' disease and in control subjects. In the patient groups with Graves' disease, serum NMP was negatively correlated with sFas (r = -0.612, p < 0.001) and serum sFas was positively correlated with FT4 (r = 0.360, p < 0.05) and TRAb (r = 0.384, p < 0.05). Serum NMP was correlated with sFas. These results suggest that serum NMP is decreased in patients with untreated Graves' disease, and that cell death or apoptosis in patients with Graves' disease is affected by soluble Fas under the influence of thyroid function.