© 2022 by Türkiye Klinikleri.Objective: In this study, the relationship between the MDR1 gene and ankylosing spondylitis (AS) was investigated. Genetic and environmental factors are known to play an important role in the pathogenesis of AS. It is obvious that different patients have different reactions to prescribed drugs and drug dosages during standard treatment. One of these factors is the P-glycoprotein encoded by the MDR1 gene. The most widely studied alleles of this gene are rs1045642-NM_000927.4:c.3435T>C, p. Ile1145Ile (C3435T), rs2032582-NM_000927.4:c.2677T>G/A, p. Ser893Ala/Thr (G2677T/A) and silent rs1128503-NM_000927.4:c.1236T>C, p. Gly412Gly (C1236T). Material and Methods: The study group was formed from 34 patients with biological therapy, 32 patients with other treatments (66 AS patients in total) and 32 healthy individuals. DNA isolation was performed using a High Pure PCR Template Preparation Kit. RNA isolation was performed with TRIzol manual isolation. For quantitative real-time polymerase chain reaction, a Roche LightCycler FastStart DNA Master device HybProbe was used. Results: There was no statistically significant difference between groups regarding the investigated polymorphisms. In the biological therapy [tumor necrosis factor alpha (TNF-α) inhibitor] group, a significant decrease was found in MDR1 gene expression (p<0.001). Conclusion: In conclusion, we could say that there is no relation between single-nucleotide polymorphism and treatment choice and response for AS patients. Decreased MDR1 expression can be explained by the possible downregulatory effect of TNF-α inhibitors.