Since its synthesis in the 1930s and subsequent introduction, sulfasalazine has been an effective treatment for inflammatory bowel disease. However, up to one-third of patients are unable to take the drug because of severe intolerance. The finding in 1977 that the anticolitic effect of sulfasalazine lay in its 5-aminosalicylic [(5-ASA); mesalazine] moiety led to the development of new generations of 5-ASA agents.These new agents include a slow continuous release formulation, pH-dependent release formulations, formulations using alternative carrier molecules and rectally administered formulations. Newer 5-ASA formulations are more effective than placebo in maintaining remission of ulcerative colitis. They have also been used for the treatment of active Crohn's disease as well as maintenance treatment of ileocolonic Crohn's disease, although their role in isolated small bowel disease is controversial. In general terms, all of the newer 5-ASA preparations are much better tolerated than sulfasalazine. The use of standard dosages of mesalazine in pregnancy appears to be tolerated; however, continuing surveillance of pregnancy outcome is recommended. While there is evidence that mesalazine can cause nephrotoxic reactions, these reactions can occur with all 5-ASA-containing preparations, particularly in individuals with existing renal disease. Blood dyscrasias can also occur with all aminosalicylates.