eNOS T786C and G894T polymorphisms affect clinical phenotype of Sickle Cell Disease according to beta globin gene genotype. 786CC genotype predisposes Sickle Cell Disease patients with S/S and β0/S genotype to proliferative retinopathy. 894TT genotype is associated with higher hematocrit in Sickle Cell Disease patients of S/S and β0/S genotype. Peripheral blood eNOS mRNA levels are not associated with beta globin gene genotype or eNOS T786C genotype.