The effect of cholesterol on the properties of phospholipid membranes. I. Structural features. Abstract: All-atom Monte Carlo simulations of four different fully hydrated dimyristoylphosphatidylcholine (DMPC)/cholesterol mixed bilayers have been performed at physiological conditions (i.e., 37^oC and 1 atm). The composition of the different samples has been chosen from both sides of the DMPC/cholesterol miscibility gap; the mole fraction of cholesterol was 0, 0.04, 0.08, and 0.40 in the four systems simulated. The configurations obtained are analyzed in detail, in order to shed some light to the role played by the presence of the cholesterol molecules in the structure of such membranes. It is found that the increase of the cholesterol concentration leads to a decrease of the average area per headgroup, and also to the decrease of the density of the membrane in the crowded region of the headgroups. However, the density in the middle of the membrane is found to be higher when considerable amount of cholesterol is present in the system. Consistently, the structure in the middle of the membrane is found to be more isotropic, and the two membrane layers are found to approach each other closer in the cholesterol-rich system than in the pure DMPC membrane or in the systems of low cholesterol content. The DMPC molecules, located next to a cholesterol molecule are found to be more ordered than the ones being far from cholesterols. However, this ordering effect of the rigid cholesterol rings on the DMPC tails is found to decrease with increasing cholesterol concentration. As a result, the overall ordering of the DMPC molecules is found to be only rather weakly sensitive to the amount of cholesterol present in the membrane.