Monthly intravenous cyclophosphamide (IVCY) has been a recommended therapy for severe lupus nephritis or neurological flare-ups in lupus patients. But the optimal treatment regimen and duration remains unknown. We report our experience in an open study of 37 patients treated with monthly IVCY. Thirty-four women and 3 men, mean age 35.5 with a mean disease duration of 59 months, with a mean 5.7 ACR criteria for SLE were analysed. 27 (group I) had lupus nephritis (OMS Class III or IV) and 10 had neurological involvement (group II). In group I, after six months of IVCY, a significant improvement was noticed in the UCH-Middlesex clinical index (2.9 pts vs 7.8), the proteinuria (3.12 g/d vs 5.4), complement and split fractions (CH50 98.4 vs 48.9%; C3 877 vs 600 mg/l; C4 177 vs 128 mg/l), the level of anti-DNA antibodies (67.5 vs 775 UI/ml) and the daily dose of steroids (22 vs 44 mg/d). Kidney biopsies showed a reduction of the activity index despite a slight increase of the chronicity index (4.1 vs 6.3 pts and 5.5 vs 3.6 pts). Those results were not maintained at medium and long term. Moreover five patients presented with worsening of renal function during IVCY treatment and two patients relapsed after the end of the treatment. In group II significant improvement was noticed at six months concerning the clinical index (1.77 pts vs 7.17) and the daily dose of steroids, 3 patients died because of cerebral vasculitis refractory to IVCY. Adverse effects are frequent: infectious (25 among 20 patients), hemorrhagic cystitis (2 events in 1 patient), gastrointestinal side effects were common (12/37 patients). Were also noticed: neutropenia (5/37), transient amenorrhea (4/28), drug induced menopausis (2/28). Overall mortality is important (7/37), uneffectiveness of IVCY was noticed in 5 patients, flares occurred in 8 patient during or after stopping treatment. IVCY seems efficacious if given at the very beginning of the flare. Its usefulness is obvious at six months among clinical and biological data in patients with severe lupus nephritis or neurological flare. It seems that long term outcome on the renal function is not modified.