Introduction. An important pathogenetic link in systemic inflammation in ulcerative colitis (UC) is dysregulation of the immune response. The aim of the study: to study pathogenetic changes in the serum level of interleukin (IL)-4 during UC therapy. Methods. 66 patients with ulcerative colitis were examined, of which 21 patients received genetically engineered biological drugs (GEBD), and 20 healthy volunteers. The amount of IL-4 was determined by enzyme immunoassay. Results. The content of IL-4 in the serum of patients significantly exceeded the values of the control group. Minimum cytokine values (3–4 pg/ml) were determined in 5 patients with severe UC activity and total intestinal lesions without GEBD therapy. Negative correlations between IL-4 values and the location of colitis and the severity of exacerbation were revealed. Achieving remission was accompanied by an increase in serum IL-4, which was less during GEBD therapy. Conclusion. A study of the IL-4 content in the blood serum of patients in the dynamics of UC revealed significant quantitative changes at different stages of the disease, exceeding the values of healthy volunteers. Restriction of overproduction of IL-4 as an anti-inflammatory cytokine can serve as a criterion for the effectiveness of therapy.