A 72-year-old man with end-stage kidney disease who began receiving continuous renal-replacement therapy in the intensive care unit (ICU) was noted to have green-colored dialysis effluent and plasma. Seven hours earlier, the patient had presented to the emergency department with a gastrointestinal perforation. An exploratory laparotomy was performed, and a gastric perforation caused by a previously unknown gastric adenocarcinoma was identified. A 40-ml solution of 0.1% methylene blue was injected into the tissue surrounding the gastric tumor to identify the lymph nodes draining the region. The gastric tumor and 11 lymph nodes were subsequently excised. Postoperatively, the patient was admitted to the ICU for the treatment of septic shock. Four hours after the initiation of continuous renal-replacement therapy, the green color was noted in dialysis effluent (Panel A) and in plasma (Panel B) that had been obtained from the patient. A diagnosis of fluid discoloration owing to the intraoperative use of methylene blue was made. Methylene blue may discolor skin or body fluids as a benign side effect of its use. Twenty-four hours after the initiation of continuous renal-replacement therapy, the green discoloration had abated. After a 1-month hospital stay, the patient was discharged home. Immunotherapy was later initiated for the treatment of liver metastases.