Pentoxifylline was shown to prevent sickling induced by deoxygenation of SS-genotype blood from sickle cell disease patients. It also prevents development of cell stiffness, based on decreased membrane fluidity. This effect is related to increased red cell ATP content, decreased potassium loss during sickling and decreased attachment of hemoglobin to the red cell membrane during sickling. There was no effect on osmotic fragility or mean corpuscular volume. There was some increase in pH of blood from patients in vasoocclusive crisis of sickle cell disease under the effect of pentoxifylline. The consideration is discussed that pentoxifylline may prevent vasoocclusive crisis, but may not abort an existing process.