Background; Sitosterolemia is a rare, autosomal-recessive lipid storage disease characterized by xanthoma, premature atherosclerosis, and hematologic abnormalities. The purpose of this study was to investigate the effect of adenosine triphosphate-binding cassette genes (ABCG) 5 and 8 polymorphisms on sitosterolemia phenotypes in a group of Turkish patients. Methods: This cross-sectional study included 90 individuals with premature atherosclerosis (n=29), xanthoma (n=32), or hematologic abnormalities (n=29). All underwent sterol level measurement by gas chromatography. All 90 individuals and 157 age-matched healthy controls were tested for two polymorphic variants (c.1895T>C and c.161A>G) of the ABCG8 gene and one polymorphic variant (c.1810C>G) of the ABCG5 gene. Results: There were no significant differences in age or gender between patients and healthy controls (p>0.05 for both). The frequencies of polymorphisms in the ABCG5/G8 gene were not significantly different between patients and controls (p>0.05). The rate of CC genotype in the ABCG5 gene in patients with xanthoma and premature atherosclerosis was 1.7 to 2.1 times higher than in patients with hematologic abnormalities. The rate of AA genotype in the ABCG8 gene in patients with hematologic abnormalities was 3.2 times higher than in patients with xanthoma. In patients with hematologic abnormalities, the mean hemoglobin and platelet values were significantly lower in patients with CC genotype than other genotypes of the ABCG8 gene. Conclusion: In Turkish patients with sitosterolemia phenotypes, the frequency of common ABCG5/G8 gene polymorphisms is similar to that of healthy controls. Our findings suggest that some polymorphisms in the ABCG5/G8 gene may be linked to sitosterolemia phenotypes (premature atherosclerosis, xanthoma, and hematologic abnormalities (specifically, hemolytic anemia and macrothrombocytopenia)).