A number of epidemiologic studies have provided evidence for an increased blood viscosity in hypertensive patients. Increased viscosity could result either from hemoconcentration, thus constituting a secondary phenomenon, or, alternatively, result directly from increased intracellular calcium concentrations in erythrocytes. The latter would augment the aggregating potential of these cellular blood compounds. This currently hypothetic view remains to be elucidated. Enhanced viscosity, however, may result in increased peripheral resistance and lead to hypertensive complications. The evaluation of antihypertensive therapy should therefore take possible effects upon blood viscosity into account.