Mycophenolate mofetil (MMF) significantly reduces proteinuria in experimental model of human membranous nephropathy (Heymann nephritis). Twenty consecutive SLE patients with persistent isolated severe proteinuria and/or proteinuric flare were studied for 18 months of MMF therapy. All of them presented stable renal function and 12 had biopsy proven membranous glomerulonephritis (WHO class V). The starting daily dose for MMF was 1.5 g to a maximum of 3 g. Patients were divided into: partial response, >or=50% decrease of baseline proteinuria; complete response, normal proteinuria levels (less than 0.3 g/24 h); flare, increase of at least 50% of the mean baseline proteinuria. All 20 SLE patients (100%) presented a 50% reduction of baseline proteinuria which was achieved in 8.2+/-3.3 months of MMF therapy, at a mean daily dose of 2.3+/-0.5 g. A significant decrease in 24-h protein excretion was observed compared to entry (3.47+/-1.26 vs. 1.33+/-0.67 g, P<0.0001) as well as a correspondent increase of serum albumin (3.2+/-0.4 vs. 3.7+/-0.4 mg/dl, P=0.02) and reduction of prednisone dose (33.7+/-20.0 to 18.6+/-14.1 mg/day, P=0.01). Complete response was observed in 11 SLE patients (55%) in 12.2+/-3.0 months of therapy with a significant decrease in proteinuria (P<0.0001), prednisone dose (P<0.0001) and an increase of serum albumin (P=0.003). Interestingly, initial proteinuria or serum albumin levels did not identify patients with complete response and those with partial response at the end of the study (P=0.543 and 0.657, respectively). Our pilot prospective study suggests that MMF appears to be effective in reducing severe persistent proteinuria in lupus glomerulonephritis, even in patients unresponsive to other immunosuppressive treatments.