Mesalazine is a 5-aminosalicylic acid compound that is the primary treatment for mild-to-moderate ulcerative colitis. In both oral and topical formulations it has demonstrated efficacy in both induction of active colitis and maintenance of remission, regardless of the extent of inflammation. In addition, there is indirect evidence of a role in the chemoprophylaxis of colorectal cancer in these patients. Mesalazine is generally well tolerated by patients, although serious adverse effects have been reported. In particular, worsening of colitis, interstitial pneumonitis and nephritis are of concern to clinicians. Fortunately these reactions are mostly reversible with cessation of therapy.