To study the effect of continuous ambulatory peritoneal dialysis on nucleoside levels and clinical course in a patient with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE). Patient We studied a patient with genetically verified MNGIE, who prior to treatment had lost weight progressively, developed amenorrhea, vomited multiple times daily, and had abdominal pain. Intervention The patient was treated with peritoneal dialysis for 3 years, and the effect on symptoms and plasma concentrations of thymidine and deoxyuridine were monitored. Dialysis stopped vomiting and reduced abdominal pain, and the patient gained 5 kg in weight and started to menstruate again. Symptoms returned if dialysis was paused. Dialysis did not affect plasma nucleoside levels. This study shows an unambiguous clinical benefit of peritoneal dialysis on gastrointestinal symptoms in MNGIE. Dialysis did not affect nucleoside levels, indicating elevated thymidine and deoxyuridine levels are not solely responsible for the pathogenesis of MNGIE.