The effect of a binifibrate (Biniwas Retard, Wasserman) on the plasma lipids and hemorheological profile of 30 primary hyperlipemic patients was studied. Our results indicate that the patients under study had evident rheological alterations as well as the expected lipid alterations. Treatment with Biniwas (2 x 550 mg/day) for six months not only substantially improved the alterations in the lipid balance but also tended to normalize the patients' hemorheological alterations, and there was a statistically significant correlation between the two effects. Apart from the decrease in plasma viscosity (1.20 +/- 0.05 vs 1.29 +/- 0.07 mPa.s, p < 0.001), the most noteworthy effects of Biniwas treatment were the decrease in red blood cell aggregability (8.7 +/- 1.2 vs 9.3 +/- 1.1, p < 0.05) and increased deformability (55 +/- 3 vs 47 +/- 5%, p < 0.001). Both changes may be due to modifications in the lipid composition of the erythrocyte membrane due to cell-plasma lipid exchange.