Irritable bowel syndrome (IBS) is a common chronic gastrointestinal disorder, which represents an economic burden to society and considerably reduces the quality of life of patients. In a previous study, the density of serotonin cells in the ileum of IBS patients was lower compared with control subjects. The present study aimed to further investigate the immunoreactivity intensity of serotonin and serotonin‑selective reuptake transporter (SERT) in the ileum of IBS patients. A total of 98 patients (77 females and 21 males; mean age, 35 years; range, 18‑66 years), which fulfilled Rome III Criteria for IBS, were included in the study. This cohort included 35 patients with diarrhoea‑predominant (IBS‑D), 31 patients with mixed diarrhoea and constipation (M‑IBS) and 32 patients with constipation‑predominant (IBS‑C) symptoms. A total of 27 subjects were included as controls (16 females and 11 males; mean age, 52 years; range, 20‑69 years). Ileal biopsy specimens were immunostained using the avidin‑biotin (ABC) complex method for serotonin and SERT. The immunoreactivity intensity was quantified by computerised image analysis using Olympus cellSens imaging software. No statistical difference of serotonin immunoreactivity intensity was identified in multiple comparisons between controls, IBS‑total, IBS‑D, IBS‑M and IBS‑C. The SERT immunoreactivity intensity was significantly increased in IBS patients as compared with controls, regardless of the subtype. It was concluded that the increase in ileal epithelial content of SERT increases the intracellular uptake of serotonin and its degradation in the gut epithelial cells and consequently decreases the availability of serotonin within the gut mucosa. The low availability of serotonin at its receptors occurred in all IBS subtypes. This may indicate that this abnormality is associated with a common symptom in all IBS subtypes, namely abdominal pain/discomfort. Serotonin acts upon sensory neurons in the submucosal and myenteric ganglia, as well as in the spinal cord, which is in agreement with this hypothesis.