Erythrocyte hemoglobin A₂ (Hb A₂)was quantitated in 28 hyperthyroid patients prior to antitnyroid therapy and at intervals during therapy. Fetal hemglobin (Hb F) levels and red cell mean corpuscular volume (MCV) were also monitored. Before therapy, Hb A Was Significantly elevated (mean ± SD, 3.3 ± 0.5%: normals, 2.5 ± 0.3%: p < 0.001). Hb F levels exceeded 1.0% in 10 cases. A tendency to red cell microcytosis was also observed (mean MCV 81.1 fl). To-date 10 of these subjects have had adequate studies following appropriate and sustained responses to antithyroid therapy. All shed a fall in Hb A₂, usually accompanied by a rise in red cell MCV. A hematologic profile for hyperthyroidism is proposed.