Rationale: Irritable bowel syndrome (IBS) is a multifactorial disease, the genetic aspect of which is being actively studied.Aim: To investigate functional polymorphism of the serotonin reuptake transporter (SERT) SLC6A4 gene of various clinical variants of IBS.Materials and methods: We performed a cross-sectional single center study in 79 Caucasian patients with IBS (according to the Rome criteria IV). The patients were divided into two groups: group 1, IBS with diarrhea (IBS-D, n = 45) and group 2, IBS with constipation (IBS-C, n = 34). The control group included 59 Caucasian patients with gastrointestinal disorders without IBS. Polymorphism 5-HTTLPR of the SLC6A4 gene was assessed in all subjects. In group 1 patients, blood serotonin levels were measured and psychological tests were performed, including Spielberger's State / Trait Anxiety Inventory, quality of life by SF36 and GSRS, Asthenia scale, VAS scores for pain intensity.Results: Thirty-five of 45 (77.8%) patients with IBS-D carried the mutant S allele, which was significantly more frequent than in the IBS-C group (p = 0.002) and in the control group (p = 0.005). There were no statistically significant differences (p = 0.54) in the frequency of detection of the homozygous LL genotype (normal allele) and the heteroand homozygous mutant alleles (SL and SS) genotype between the IBS-C and control patients. In the IBS-D group, a gender difference for the mutant SS allele of 5-HTTLPR was found, with significantly higher frequency in female patients (p = 0.0147). No significant gender differences in the genotype distribution between the patients with IBS-C and the control group were found. There were also no differences in blood serotonin levels in the IBS patients with various 5-HTTLPR types (p = 0.086); they were all in the reference range. However, there was a trend towards lower serotonin levels in the LL genotype carriers compared to those with the SS/SL polymorphisms. The Gastroenterological inventory GSRS demonstrated significantly higher total score for the constipation syndrome in the patients with homozygous LL 5-HTTLPR polymorphism, compared to that in the patients with the SS/SL genotype (p = 0.013).Conclusion: The results may be related to lower expression of the SLC6A4 gene in the carriers of the mutant allele in the 5-HTTLPR promoter and subsequent decreased rate of serotonin uptake, with resulting stimulation of the gastrointestinal tract. The SERT polymorphism of the SLC6A4 gene is worth further investigation as a potential candidate gene in the IBS pathophysiology.