Azathioprine is an efficient maintenance treatment of IBD, able to maintain a complete clinical and anatomical remission in about one third of patients. However there are concerns regarding its long term tolerance, particularly myelosuppression and malignancy. Azathioprine is not required in about one third of Crohn's Disease patients and more than half of Ulcerative Colitis patients who will experience a mild disease course. In patients with more severe disease, although anti-TNF agents are more powerful and act more rapidly, there is a subset of patients with moderate-to-severe IBD without important anatomical damage who may achieve a prolonged steroid-free clinical and anatomical remission on azathioprine monotherapy. It is thus advised to initiate azathioprine monotherapy in these intermediate cases, and to continue azathioprine if anatomical remission is achieved.