Erythrocytes have to constantly adapt themselves to the varying circulatory system shear stress forces and capillaries diameter. Membrane lipid and protein content have an important role in determining the erythrocyte shape and are main determinants of the membrane solid and fluid behaviour which enables the erythrocyte to respond to the outer environment modifications. Membrane fluidity is an inverse index of membrane microviscosity. The aim of the present work is to evaluate prospectively in three periods of time (discharge, after 6 months and one year later) in survivors of an acute myocardial infarction (AMI) the erythrocyte membrane fluidity (outer and inner bilayer) and establish a relation with the cardiovascular events or need of coronary revascularization during a two year clinical follow up. Sixty survivors of acute myocardial infarction were recruited during 1994-96 and were prospectively studied in three periods (discharge, 6 months and after one year), and were compared with a control group (n = 36). Membrane lipid fluidity was determined by means of fluorescence polarisation with two probes: 1,6-diphenyl-1,2,5-hexatriene (DPH) and 1,4-trimethylamine 6-phenyl hexa-1,3,5-triene (TMA-DPH), for the characterisation of the hydrophobic and external polar region, respectively. The hydrophobic region was more rigidified (p < 0.01) in the erythrocytes from AMI patients, in relation to the control group. During the time of the study there was a progressive erythrocyte membrane rigidification (DPH p < 0.001; TMA-DPH p < 0.001). We found no relation between erythrocyte membrane fluidity and the coronary risk factors, cardiovascular events or the need of coronary revascularization during the clinical follow-up. In conclusion, after the myocardial infarction erythrocyte membrane of AMI survivors becomes more rigid with time, which could contribute to the decreased erythrocyte deformability and the increased blood viscosity previously described in this group of patients.