The thyroid hormone titres of the sera of 25 euthyroid infiltrative ophthalmopathic patients was examined, TRH test was performed, and thyroid-stimulating antibodies were studied by membrane receptor assay and TRAK assay. Previously, other diseases causing exophthalmos could be excluded by ophthalmological, radiological examinations, orbital ultrasonography and/or CT. Following TRH administration, 18 out of 25 patients showed abnormal TSH response, 16 of them were TSI - positive. Five of them became hyperthyroid 2-2.5 years later. After TRH administration 7 patients produced normal TSH response, none of them became hyperthyroid in the subsequent 2-4 years follow-up period. In the 7 TRH-negative patients, four were found to have higher hTG and an antithyroid microsome antibody titre. In those patients the fine needle biopsy verified chronic lymphocytic thyroiditis. In three patients there was no evidence of a pathological change of the thyroid gland. Based on our results, the patients could be divided into three groups. The prognostic and therapeutic differentiation of these groups seems to be justified.