Idiopathic thrombocytopenic purpura is an infrequent extraintestinal manifestation of inflammatory bowel disease. It is particularly rare in association with Crohn's disease. In these cases it presents certain peculiarities with respect to its normal clinical features, occurring more frequently when Crohn's disease is mainly colonic and usually appearing after the diagnosis of Crohn's disease has been made. Moreover, it usually complicates the clinical course of both diseases. We present a case of idiopathic thrombocytopenic purpura in a 14-year-old girl in whom the two diseases first presented simultaneously. The clinical course was poor and both diseases were refractory to habitual treatment.