Plasma viscosity (Pv), and that of an erythrocyte suspension in isotonic saline solution (called erythrocyte viscosity--Ev) were determined by means of a viscosimeter of personal construction. The Ev/Pv ratio, called relative erythrocyte viscosity (Rv), was then calculated. The measurements were performed in flow conditions similar to the physiologic ones. An analysis of the variations of these parameters in different internal diseases has revealed high Pv values in the pathologic states associated with dysproteinemia. Pv correlates with the ESR but not with the hematocrit. Ev levels were increased in many diseases and did not correlate with any of the current laboratory parameters. Ev might be a prognostic factor for the arterial hypertension evolution. Rv analysis suggests a possible dynamic balance between Pv and Ev.