A number of immunological parameters have been monitored for up to 6 weeks following 131I treatment for hyperthyroidism in Graves' disease. The aim was to examine whether this isotope treatment normalizes or further accentuates some immunological abnormalities which may be a manifestation of autoimmune reactions in these patients. It was confirmed that both the cellular composition and immunological reactivities of the patients' blood lymphocytes were abnormal before treatment. After 131I administration a slight lymphopenia occurred and the ratio between CD4 and CD8 positive T lymphocytes (helper-inducer/suppressor-cytotoxic), which was increased before treatment, increased further. Moreover, PWM-triggered IgM secretion in vitro was reduced by 50%. No other immunological parameters studied, such as secretion of other Ig classes, mitogenic responses of lymphocytes, and distribution of other lymphocyte subsets, changed to any detectable extent. It remains speculative whether the 131I-induced changes of the immune system may further accelerate the underlying autoimmune disease processes.