1. Blood rheology has been quantified by measuring blood and plasma viscosity, packed cell volume (PCV), erythrocyte filterability and erythrocyte aggregation in forty-eight voluntary vegetarians and compared with matched controls. 2. Results show that in vegetarians, values for PCV were lower than those in controls, leading to reduced native blood viscosity. In addition PCV-standardized blood viscosity was also decreased. This was brought about mostly by lower plasma viscosity. Erythrocyte rheology seemed to be unaltered. Stricter avoidance of animal products was associated with even lower values for these indices. 3. These observations are in agreement with the fact that other low-cardiovascular-risk groups show better than average blood fluidity. They are consistent with the hypothesis that in vitro measurements of blood rheology may provide signs of early atherosclerotic changes in vivo.