Ulcerative colitis is a chronic relapsing inflammatory disease of the colon with continuous spread of lesions from the rectum in the proximal direction. The etiology of the disease is unknown, but it is assumed to be developed as a result of a combination of several factors. The clinical picture is often similar to the picture of intestinal infection, which complicates the timely diagnosis of the disease, especially in children. Refractory to standard therapy limits therapeutic options and increases the frequency of surgical interventions. However, the emergence of new drugs, such as upadacitinib, opens up new opportunities for treating such patients and studying the effectiveness of therapy. We present a case of successful treatment of ulcerative colitis with upadacitinib in a child with the onset of the disease at the age of 6 months. The purpose of the description is to show the possibilities of using the janus kinase blocker Upadacitinib in severe ulcerative colitis in a child with a very early onset of the disease and without effect from previous therapy. The importance of early diagnosis and therapy is emphasized.