Activation of T lymphocytes has been found to be associated with an increase in soluble interleukin-2 receptor (sIL-2R) levels. The aim of this study was to investigate serum levels of sIL-2R in 20 untreated patients with Graves' disease and to relate these levels to disease activity and to TSH-receptor, anti-thyroglobulin, anti-microsomal and anti-eye muscle antibodies. sIL-2R levels were significantly increased in newly diagnosed Graves' patients compared with controls (667 +/- 270 vs 205 +/- 45 U/ml) (P less than 0.001). The sIL-2R levels were higher in patients with active infiltrative ophthalmology than in those without eye symptoms (810 +/- 313 vs 525 +/- 180 U/ml). All patients were treated with methimazole for at least 12 months. sIL-2R levels were normalized by methimazole treatment in the majority of patients without ophthalmopathy but not in those with ophthalmopathy. In five patients sIL-2R serum levels were studied after interruption of thyrostatic therapy. An increase was observed in three patients and hyperthyroidism subsequently relapsed in two of these. Furthermore, a correlation was found between soluble interleukin-2 receptor levels and TSH-receptor antibodies but not with other immune parameters examined. Serum sIL-2R represents a useful marker of immunological activity in Graves' disease.