In order to investigate the immunological aspects of silent thyroiditis, T lymphocyte subsets were analyzed by two-color flow cytometry in 10 patients with silent thyroiditis in the thyrotoxic phase, 12 patients with silent thyroiditis in the recovery phase, and 11 healthy volunteers as a control. The percentages of CD3+ CD4+, and CD8+ cells were similar in all three groups. In contrast, activated matured T (HLA-DR+CD3+), activated helper/inducer T (HLA-DR+CD4+) and activated suppressor/cytotoxic T (HLA-DR+CD8+) cells were more numerous in both the thyrotoxic and the recovery phases of patients with silent thyroiditis when compared with healthy controls. In a serial study of 6 patients with silent thyroiditis, the percentage of activated helper/inducer T (HLA-DR+CD4+) cells was higher in the thyrotoxic phase than in the recovery phase. These data indicate that the activation of T cells, especially of helper/inducer T cells, might be important for the induction of silent thyroiditis.