We describe a case of a 25-yr-old woman with ulcerative colitis who developed marked thrombocytopenia during treatment and upon rechallenge with oral mesalamine. In contrast to its parent drug, sulfasalazine, which has often been reported to cause serious blood disorders, particularly agranulocytosis, mesalamine has rately been implicated as a cause of serious blood disorders. Although previous cases of hematological toxicity have been described in patients taking mesalamine, none of these patients were rechallenged in an effort to prove causality between 5-aminosalicyclic acid and the hematological abnormality as well as outrule the possible "autoimmune" contribution of inflammatory bowel disease to the hematological toxicity of these agents. This report demonstrates that mesalamine has the potential, like sulphasalazine, to cause marked thrombocytopenia in an idiosyncratic fashion. All patients receiving mesalamine therapy, either orally or topically should have regular, complete blood profiles.