The possible adverse consequences of biological therapies in inflammatory bowel diseases (IBDs) were recently highlighted in this journal by D’Haens (Gut 2007; 56: 725–32). We here describe a hitherto unappreciated adverse effect to treatment with the B lymphocyte (B cell) depleting agent rituximab (RTX), 1 namely the occurrence of ulcerative colitis and arthritis shortly after treatment with RTX. Our patient, a 45-year-old Caucasian female, had had mild irritable bowel symptoms since 1992 at which time rigid sigmoidoscopy and bowel x ray were normal. She had never received any therapy, had never previously had joint pain, and was not predisposed to IBD. In April 2005 she was diagnosed with Graves’ disease, and after uncomplicated standard methimazole therapy she received four weekly doses of 375 mg/m2 RTX from day 1 to 22 as part of a clinical trial. 2 The trial was approved by the local ethics committee and recorded on www. clinicaltrials. gov.