Ulcerozni kolitis kronična je upalna bolest za koju su karakteristične ulceracije sluznice debeloga crijeva i klinički tijek obilježen razdobljima remisije i relapsa bolesti. Klinički se ulcerozni kolitis najčešće očituje učestalim sluzavo-krvavim proljevima. Azatioprin je tiopurin koji imunosupresiju postiže putem svojih aktivnih metabolita. Oni potiču apoptozu T-limfocita, te, inkorporirajući se u replicirajući lanac DNA i blokirajući de novo sintezu purina, smanjuju njihovu proliferaciju. Mjerenjem aktivnosti tiopurin Smetiltransferaze i praćenjem metabolita tiopurina, uspješno se postiže optimizacija terapije. Time se poboljšava klinička djelotvornost liječenja, te sprječava neuspjeh liječenja uzrokovan štetnim nuspojavama azatioprina. Unatoč pretpostavkama da će u budućnosti, zbog razvoja učinkovitijih i sigurnijih lijekova, uloga tiopurina u liječenju bolesnika s ulceroznim kolitisom biti ograničenija, azatioprin i dalje ostaje neizostavan lijek u liječenju bolesnika s ulceroznim kolitisom. Ulcerative colitis is a chronic inflammatory disease characterized by ulcerative lesions of the colon mucosa and clinical course characterized by periods of remissions and relapses. The most common clinical symptom of ulcerative colitis is frequent bloody diarrhea usually accompanied by mucus. Azathioprine is a thiopurine which achieves immunosuppression through its active metabolites. They induce apoptosis of T-lymphocytes and, by incorporating into the replicating DNA chain and blocking de novo purine synthesis, reduce its proliferation. The measurement of thiopurine S-methyltransferase together with thiopurine metabolites monitoring successfully lead to therapy optimization. Thus, improving clinical effectiveness of the therapy and preventing management failure from adverse effects. Despite assumptions that in future, due to the intensive development of more effective and safer medicaments, the role of thiopurines in the treatment of patients with ulcerative colitis will be more restricted, azathioprine still remains an unavoidable medicament in the treatment of patients with ulcerative colitis.