We describe a 20-year-old female with systemic lupus erythematosus (SLE) who developed renal failure during continuous ambulatory peritoneal dialysis (CAPD). The patient was treated with mizoribine (MZR). MZR peak concentration was 1.8 mug/ml and was sufficient for clinical efficacy as measured by serological data. Treatment with MZR was safe and useful even while undergoing CAPD. Achieving optimal MZR blood concentration was important for treatment of SLE, even though the patient was in end-stage renal failure.