Idiopathic nephrotic syndrome (INS) is a common chronic illness characterized by massive proteinuria and hypo-albuminemia in children. Baseline treatment is 6 month-corticotherapy. In cases of steroid resistant/dependent INS several types of treatment are used, including course of methyloprednisolone "pulses", alkylating agents, cyclosporin A, levamisole and mycophenolate mofetil. It has been suggested that children with frequently relapsing nephrotic syndrome or steroid-dependent nephrotic syndrome had a significantly longer relapse-free period if rituximab (RTX) treatment was additionally applied. We present a case of a 4.5 boy who due to steroid-sensitive, steroid-dependent nephrotic syndrome has been successfully treated with RTX. Administration of the one dose of Rituximab in the patient caused immediate decrease of CD19/CD20 positive B lymphocyte population. The depletion of B cells has been observed for the next six months. With regard to the fact that RTX treatment may affect patient's immune response, comprehensive immunodiagnostic has been conducted in a course of the Therapy.