As reduced red cell deformability (RCD) can contribute to derangement of the microcirculation, a central feature in the pathogenesis of severe malaria, RCD was measured with a laser diffraction technique in 232 consecutive patients with falciparum malaria on the Kenyan coast, of whom 99 had severe disease. RCD on admission (measured as elongation index [EI] at shear stress = 1.7 Pa) was reduced in proportion with severity of disease (fatal outcome: EI = 0.182 (SD = 0.048), survivors from severe disease: EI = 0.217 (SD = 0.043), uncomplicated malaria: EI = 0.249 (SD = 0.030), healthy controls: EI = 0.268 (SD = 0.022). All but 2 survivors with severe malaria and rigid erythrocytes received a blood transfusion restoring RCD. Reduced RCD may contribute to impaired microcirculatory flow and a fatal outcome in falciparum malaria. RCD can be improved by blood transfusion. Since severely reduced RCD has a strong predictive value for mortality, blood transfusion possibly improves disease outcome not only through its beneficial effect on anaemia but also on RCD.