The morphology and deformability of erythrocytes and the lipid composition of the blood plasma and red cells were studied in rabbits during a diet with 1% cholesterol. After 2 weeks of diet the concentration of the atherogenic lipid lysophosphatidylcholine was significantly increased. 25% of the erythrocytes had an abnormal shape (echinocytes) and the viscosity was increased at the lower shear rates of 0.1-1/s. The shape transformation of the erythrocytes was in vitro after 2 weeks spontaneously reversible, but not after 10 weeks of diet. After 10 weeks the deformability of the erythrocytes was reduced, whereas it was normal before, despite the altered cell shape and the increased low-shear viscosity. It is concluded that the pathophysiological significance of echinocytes does not originate from a diminished deformability as reported before.