The aim of this study was to compare the clinical efficacy and adverse effects of intravenous cyclophosphamide pulse therapy between 2-week and 4-week intervals in the treatment of severe systemic lupus erythematosus. 52 patients were involved in the study and were equally divided into two groups. The pulse intervals were 2 weeks for group I and 4 weeks for group II. Each pulse dose of cyclophosphamide was 15 mg/kg. Every patient took prednisone 1 mg/kg daily for first 8 weeks and then tapered. Most of disease indices recovered quicker in group I than in group II. The group with short interval was better than that of long one on total efficacy and remission rate. The cyclophosphamide cumulative doses were not different between two groups as disease activity index dropped 50%. The pulse interval had to be prolonged from 2 weeks to 4 weeks to six patients in group I (23.1%) because of leukopenia, and be shortened from 4 weeks to 2 weeks to five cases in group II (19.2%) because of inefficacy. There was no statistical difference in adverse effects between two groups. Intravenous cyclophosphamide pulse therapy with two-weeks interval is advantageous to recovery of severe systemic lupus erythematosus provided blood test prior to each pulse. We suggest that pulse interval should be individualized.