INTRODUCTION: Chromoendoscopy combines endoscopy and staining as a diagnostic technique to identify pre- and cancerous lesions. Methylene blue, one of many stains used, is sprayed directly in the lumen. Here we report the case of a man who experienced blue urine after undergoing chromoendoscopy. CASE DESCRIPTION/METHODS: 71-year-old man with a history of UC on sulfasalazine and colon cancer presented to the endoscopy suite for colonoscopy. He was diagnosed with colon cancer 2 years ago and underwent resection and chemotherapy, with PET scan 1 year later showing no evidence of disease. On surveillance colonoscopy a few weeks prior to current presentation, a malignant polyp was found in the rectum along with significant inflammation. He presented to the endoscopy suite for chromoendoscopy to evaluate for dysplasia. It showed residual high-grade dysplasia. Shortly after, the patient's urine color turned blue. Initially, he feared he was having a stroke or seizure, but his wife confirmed indeed his urine was blue. The patient had been on the same two medications for years: atorvastatin and sulfasalazine. The only new agent was methylene blue for his endoscopy. This led us to conclude the blue dye used during chromoendoscopy is the source of the blue urine. DISCUSSION: Methylene blue has few indications, one of which is endoscopic dysplasia screening especially in cases of IBD and Barrett's esophagus. The absorption of methylene blue by the colonic lumen is a known but rare side effect. In fact, we have not found any case reports of blue urine in humans as result of chromoendoscopy. There are case reports of blue-green urine but primarily due to blue dye added to enteral feeds and intravenous administration. Many patients undergo chromoendoscopy however without any changes in their urine color. In our patient, the active inflammation could have led to enhanced absorption of methylene blue. There are a few reports to suggest that intestinal mucosa permeability is increased in IBD, possibly correlating to the degree of inflammation, which could have led to increased absorption of methylene blue and ultimately urine excretion. We conclude that our patient with active ulcerative colitis who underwent chromoendoscopy with methylene blue had increased intestinal absorption of methylene blue and urine excretion. This effect was possibly enhanced by active inflammation causing damage to the intestinal lumen leading to enhanced permeability.