Whole blood viscosity, plasma viscosity, aggregation of red cells and internal viscosity (deformability) of erythrocytes were investigated in twelve hyperthyroid patients and six hypothyroid patients. Significant differences were found in the calculated relative blood viscosity and also in the deformability of red cells in these two groups. It is suggested that the increase in relative blood viscosity and in the deformability of the red cell may, in part, account for the circulatory disturbance in hypothyroidism.