Very few studies have been published on how to treat children with membranoproliferative glomerulonephritis type I (MPGN I), and as yet there is only one report on the use of mycophenolate mofetil (MMF) in children with MPGN I. We report a 12-year-old boy who presented with edema, hypertension, nephrotic range proteinuria, and microscopic hematuria following an upper respiratory tract infection. Laboratory tests revealed a serum creatinine of 90 micromol/l, albumin of 20 g/l, and a C3 of 0.11 g/l (normal range: 0.7-1.4). Renal biopsy showed the presence of MPGN I. Upon failure to induce remission with prednisone, we started the patient on MMF at 500 mg/day (300 mg/m(2)), increasing up to a final dose of 2 g/day (1200 mg/m(2)), with a MMF metabolite mycophenolic acid (MPA) target range of 2-5 mg/l. Prednisone was subsequently reduced to alternate day therapy and gradually weaned to 7.5 mg on alternate days over 9 months. Within 4 months of starting MMF therapy, there was significant improvement in serum creatinine (decrease from 156 to 64 micromol/l), serum albumin (increase from 23 to 40 g/l), and proteinuria (decrease from 13 g/day to 40 mg/day). Twelve months following the introduction of MMF into his therapeutic regimen, he remains in remission with no further relapses. In summary, this case suggests that there may be potential benefit for use of MMF in children with refractory MPGN I, which supports the rationale for prospectively evaluating MMF treatment in a treatment trial of refractory MPGN I.