Idiopathic Myointimal Hyperplasia of the Mesenteric Veins (IMHMV) is a rare and poorly understood cause of chronic bowel ischaemia. Fewer than 130 cases are reported in the literature. IMHMV is characterised by non-inflammatory thickening of mesenteric veins with luminal narrowing, venous congestion and ischaemia. The aetiology remains unclear. IMHMV predominantly affects men in their sixth decade and most commonly manifests in the sigmoid and rectum. IMHMV can mimic Ulcerative Colitis (UC) with patients typically experiencing abdominal pain, diarrhoea, haematochezia, and weight loss. Colonoscopy findings are visually similar to UC and cross-sectional imaging commonly shows chronic segmental inflammatory changes. Case: A 66 year old male presented with several months of bloody diarrhoea. Colonoscopy showed appearances consistent with UC but histology identified an ischaemic process and raised the possibility of IMHMV. There was no response to treatment with Mesalazine and he re-presented acutely with severe diarrhoea. CT showed segmental colitis. Following two weeks of unsuccessful medical management, he underwent a laparoscopic-converted to open Hartmann’s procedure. His symptoms have resolved after surgery. Histology of the specimen confirmed IMHMV. Conclusion: IMHMV is a rare and serious cause of chronic colonic ischaemia which can be mistaken for UC. The disease course is progressive and there are no suitable medical management options; definitive treatment is resection of the affected bowel. Most reported cases of IMHMV were diagnosed post-operatively with surgery being delayed as other causes of colitis were considered. We hope to raise awareness of this condition with the first case report of IMHMV in New Zealand.