INTRODUCTION: Acute lower gastrointestinal (GI) bleeding is uncommonly due to medication induced-colitis, reported in less than 6% of all lower GI bleeds. Identifying medications that may cause GI bleeding is particularly important in chronically ill patients at high risk for morbidity and mortality. A commonly used medication in patients with advanced renal disease is sevelamer, a phosphate binder in renal failure patients. CASE DESCRIPTION/METHODS: A 53-year-old female with end-stage renal disease presented with hematochezia for 2 days. She notes dark black stools for two weeks followed by rectal bleeding. Her past history is notable for a renal transplantation in 2008 with graft failure requiring hemodialysis. She has no history of NSAID use and her prior screening colonoscopy showed no evidence of colitis. Her medication list was extensive and included sevelamer and cinacalcet. The patient was hemodynamically stable. Initial labs showed a hemoglobin of 6.0 g/dl, baseline was 8.0-9.0 g/dl. She was resuscitated with intravenous fluids and received 2 units of blood. Next, the patient’s upper endoscopy showed gastric erythema only with no stigmata of recent bleeding. Her colonoscopy showed a macroscopically normal appearing colon however random biopsies revealed expansion of the lamina propria with significant population of increased eosinophils. Several crystals were embedded within the colonic mucosa and submucosa resembling sevelamer crystals. Based on the pathology, she was diagnosed with sevelamer-induced colitis and this medication was discontinued. Follow-up at 3 months showed the patient remained asymptomatic without further GI bleeding. DISCUSSION: Sevelamer is a common medication used in renal failure patients. It is a resin that binds and prevents absorption of phosphate within the GI tract and has been shown to cause GI issues such as nausea, vomiting and diarrhea. Rarely it can crystalize, become embedded in the GI tract and ulcerate and cause bleeding. A few case reports have described rectosigmoid ulceration and pseudotumor presentation of sevelamer induced colitis.1,2 Understanding and recognizing the potential for sevelamer-induced colitis is important with such a widely used medication.