The C677T methylenetetrahydrofolate reductase (MTHFR) gene polymorphism and the G20210A mutation at the 3' untranslated region (3'UTR) of the prothrombin gene may be considered to be genetic risk factors that contribute to the clinical heterogeneity in sickle cell disease. The current study investigated a group of sickle cell (SS) patients from Salvador-Bahia, Northeast Brazil in order to determine the prevalence of these polymorphisms, using the polymerase chain reaction (PCR) and restriction fragment length polymorphim (RFLP) techniques. Out of 69 SS patients diagnosed with the C677T MTHFR gene polymorphism, 13 (18.6%) were heterozygous and four (5.7%) homozygous. The G20210A mutation was not found in 50 SS patients investigated. These results became important once the C677T MTHFR gene polymorphism was found to be an independent risk factor for vascular disease, a common clinical event in sickle cell disease.