The percentage of total and "activated" peripheral blood T lymphocytes was measured in patients with (i) hyperthyroid Graves' disease selected for absence of ophthalmopathy, (ii) treated Graves' disease, with ophthalmopathy, and (iii) other thyroid disorders including subacute thyroiditis, using sheep red blood cell rosette tests. No patient had significantly increased levels of either total or "activated" T lymphocytes. On the other hand, the percentage of total T cells was below normal in 8 of 18 patients with hyperthyroid Graves' disease and in 6 of 18 patients with ophthalmopathy compared with only one of 12 patients with nodular goitres. Similarly, low levels of "activated" T cells were demonstrated in 5 of 18 patients with hyperthyroid Graves' disease and in 10 of 18 patients with ophthalmopathy compared with only one of 12 patients with nodular goitres. Three of four patients with subacute thyroiditis tested had depressed levels of total T lymphocytes whilst only one had low levels of "activated" T lymphocytes. Levels returned to normal during the recovery phase in the two patients with positive tests who were retested. Depressed levels of T lymphocyte populations in patients with Graves' disease and subacute thyroiditis may be due to feedback suppression and/or "exhaustion" in association with the thyroidal and orbital immunological reactions.