In patients with inflammatory bowel disease (IBD) there is an increased incidence of thromboembolic events. We report a case of a female, age 14, with a 6 months history of diarrhea with occasional presence of red blood and with a very distended abdomen with evident ascites. The diagnosis was Budd-Chiari syndrome (BCS) in ulcerative colitis (UC). Therapy with subcutaneous low molecular weight heparin, methylprednisolone, mesalazine was started. Clinical and radiological features quickly improved. No thrombophilia abnormality nor other risk factor for thrombosis were detected. BCS is a rare condition caused by obstruction of the supra-hepatic veins and causes liver congestion, portal hypertension, ascites, esophageal varices, and in some cases, acute onset of severe liver failure. Only six pediatric cases of BCS with concurrent UC are reported. Of these, four had no thromboembolic risk factor. IBD should always be carefully evaluated as a possible underlying trigger of an acute thrombotic event.