Immunosuppressive drugs have become a mainstay of therapy for the inflammatory bowel diseases. Although robust evidence exists in support of the use of these drugs in Crohn's disease, a close evaluation of the available data in ulcerative colitis reveals a much weaker evidence base. In particular, randomised controlled trials of azathioprine, the most commonly used immunosuppressive agent, do not provide rich evidence of efficacy whereas observational cohorts suggest this agent is effective, particularly in patients with relapsing disease who require corticosteroids. Ciclosporin is also effective in the most refractory cases but its efficacy needs to be carefully weighed against the possibility of rare but life threatening complications. Although the evidence base in support of immunosuppressive drugs in ulcerative colitis is not as strong as in Crohn's disease, these agents clearly have a role in the treatment of this disease.