A 29-year-old male presenting nephrotic syndrome and facial skin erythema was admitted to our hospital in September of 2000. We diagnosed him as having systemic lupus erythematosus (SLE) accompanied by lupus nephritis (WHO class V). The disease activity had decreased after treatment with methylprednisolone (m-PSL) pulse therapy, which was followed by oral PSL. Thereafter, when tapering the dosage from 60 to 30 mg/day, the lupus nephritis flared up and he was re-hospitalized in February of 2001. After successful retreatment with m-PSL pulse therapy followed by the tapering of the dosage from 60 to 30 mg/day, we used mizoribine (MZR) as a combination therapy. The lupus nephritis flared up again after tapering down to 17.5 mg/day of PSL. Then, we changed the MZR dosage from 150 mg/day in three divided daily doses to 200 mg/day in two divided daily doses. This modification increased the peak blood concentration (Cmax) of MZR from 0.63 to 1.55 microg/ml. At present, we have been able to successfully taper the dosage to 7.5 mg/day of oral PSL and the patient has achieved a state of remission without any side effects. Monitoring of the serum concentration of MZR is thus considered to be important for achieving effective therapy of SLE, especially for steroid-resistant lupus nephritis. If the serum concentration of MZR does not reach an effective level, then the dosage of MZR should be adjusted appropriately in order to maintain an adequate serum concentration of MZR.