The relationship between hematocrit and blood viscosity was investigated in fresh blood samples obtained from a control group of normal persons and from a group of patients with burns. Barras' equation describing the percent change in blood viscosity caused by a change of 1% in the hematocrit was found useful for the quantitative assessment of the relationship between whole blood viscosity and hematocrit. Although individual variations were observed, the average change in viscosity per 1% hematocrit change was identical in the control group and the burn patients, the value being close to that reported by Barras. Analysis of covariance showed an increased blood viscosity independent of the hematocrit in the burned patients as compared with the controls.