What is the cause of ischemic colitis? The obvious answer is a reduction in the blood supply to the colon. But does this occur because of hypotension, arterial disease, arterial emboli, vasculitis, small vessel disease or venous thrombosis? In a minority of patients, ischemic colitis is clearly related to hypotension associated with cardiac disease, serious illness or recent surgery. In most patients however, the cause remains unclear even after extensive investigations including mesenteric angiography. In the patient described below, ischemic colitis was attributed to thrombosis in a segmental branch of the inferior mesenteric vein. This may be the first reported case of reversible mesenteric venous thrombosis that has been confirmed by imaging techniques. The patient was a 50-year-old woman who was referred for evaluation because of lower abdominal pain and rectal bleeding for 12 hours. Prior to the development of symptoms, she described intermittent constipation and had previously had a hysterectomy. On examination, there was mild tenderness on palpation over the lower abdomen. Blood tests revealed a mild elevation of her white cell count and C-reactive protein. Plain X-rays of her abdomen were normal but an abdominal ultrasound study revealed segmental thickening of the wall of the sigmoid colon. A contrast-enhanced computed tomography (CT) scan on the second day after admission showed a tubular defect of low attenuation within the sigmoid vein (Fig. 1, arrow). At colonoscopy, on the fourth day after admission, the sigmoid colon was abnormal with mucosal edema, patchy erythema, contact bleeding, nodularity and minor ulceration (Fig. 2). The appearance was typical of ischemic colitis and this was supported by histological evaluation of sigmoid biopsies. Her symptoms resolved over 3 days and follow-up investigations including a barium enema study, repeat colonoscopy and a repeat CT scan were normal apart from linear scars in the sigmoid colon at colonoscopy. Extensive investigations for a hypercoagulable state were undertaken but no abnormalities were detected. Serological studies raised the possibility of a connective tissue disorder but there were no clinical features and the patient was well on follow-up after 15 months.