Idiopathic nephrotic syndrome is steroid-sensitive in more than 90% of cases in children. However 60% of cases are steroid dependent and required treatment with immunosuppressive agent. Cyclophosphamide and ciclosporin are used for long time to reduce steroid dependency, but duration of these treatments should be restricted because of gonadotoxicity for cyclophosphamide and nephrotoxicity for ciclosporin.

Mycophenolate mofetil appears as an alternative treatment without gonadotoxicity and nephrotoxicity. However, contrary to cyclophosphamide, mycophenolate mofetil does not seem to have a residual action so that treatment must be maintained during months or years.

The aim of the study is to compare efficacy of cyclophosphamide and mycophenolate mofetil in steroid dependent nephrotic syndrome in children.

Aim of the study: Comparison of efficacy of cyclophosphamide 148mg/kg in 12 weeks and mycophenolate mofetil 1200mg/m² during 18 months, in children with steroid dependent nephrotic syndrome.

The 70 patients will be recruited in the 26 centres of paediatric nephrology in France, included and randomized at the time of a relapse. They will receive the same steroid treatment in the 2 arms.

The primary point will be occurrence of a relapse during the 24 months of follow-up. Detection of relapse will be done by using dipsticks and confirm by biological dosages (albuminemia and proteinuria/CREATININURIA ratio). Clinical and biological check up will be done every 3 months during all the study.

Inclusion Criteria:

• children 2 to 16 years old
• steroid dependency ≥30mg/m² eod
• or steroid dependency ≥15mg/m² eod and occurrence of : at least 2 relapses in 1 year, adverse event of steroid therapy (height rate ≤-1SD, obesity, other complication) or severe complication of nephrotic syndrome (thrombosis, collapse, severe infection,…)
• inform consent

Exclusion Criteria:

• steroid resistant nephrotic syndrome
• prior treatment with cyclophosphamide, mycophenolate mofetil or cyclosporine
• absence of contraception in pubescent girls
• allergy to cyclophosphamide or mycophenolate mofetil
• malignant disease
• treatment with other immunosuppressant treatment or with non-steroid anti-inflammatory or anti proteinuric medication (enzyme converse antagonist and angiotensin II receptor antagonist)
• absence of inform consent
• participation to other therapeutic trial