: We have examined neutrophil adherence, chemotactic activity, and random migration in 35 hyperthyroid patients with Graves' disease and 106 normal volunteers. No statistically significant differences were found between granulocyte adherence of 17 hyperthyroid subjects (67 ± 15.6%) and 81 healthy volunteers (63.1 ± 17%). In 3 thyrotoxic patients, impaired neutrophil adherence was found, which resolved when thyroid function returned to normal. The neutrophil chemotactic activity of 32 normal controls was 107.5 ± 21.4 cells, and the random migration 36 ± 15.5 cells. No statistically significant difference was demonstrated in 13 hyperthyroid patients who had a neutrophil chemotactic activity of 102 ± 14.6 cells and a random migration of 31.2 ± 13.2 cells. Defective chemotactic activity and random migration was found in 2 patients. Neutrophil functions returned to normal in one of the two subjects who were re-evaluated when thyroid function recovered. In summary, 14% of hyperthyroid patients had impaired leukocyte functions. However, severe pyogenic infections are quite rare in hyperthyroid patients, indicating that the observed alterations in function of phagocytic cells are not clinically important.