The primary purpose of the study is to identify the factors of steroid dependency in childhood idiopathic nephrotic syndrome. The steroid dependency is defined by a relapse of nephrotic syndrome within the 3 weeks that follow the withdrawal of steroid therapy after the first manifestation. Different clinical and biological factors will be analyzed: age of first manifestation, delay of remission, ethnicity, and preceding viral infection, geolocalization in the Parisian area, genoprevalence of herpes viruses and polymorphisms in the genes involved in the response to steroid therapy.

Patients with a steroid dependent nephrotic syndrome will be compared to those with a non steroid dependent form of nephrotic syndrome at the end of the first manifestation. An additional blood volume will be sampled in patients and controls during a scheduled biological check-up for the initial disease. Genetics factors will be checked using a DNA chips devoted to the response to steroid therapy and viral genome of EBV, CMV, HHV7 will be searched for using PCR reaction in total blood DNA extract.

Inclusion Criteria:

• children below 16 years
• first manifestation of nephrotic syndrome
• steroid sensitivity according to the recommendations of the french "pediatric society nephrology"

Exclusion Criteria:

• patients who don't stay in Paris during the study

Any children below 16 years of age with a first manifestation of idiopathic nephrotic syndrome commencing or having commenced in the Paris area