Minimal change nephrotic syndrome (MCNS) is the most frequent type of nephrotic syndrome (NS) in childhood. During the last decade only little progress has been made in the understanding of the etiology and pathogenesis of MCNS. However, treatment has been improved pragmatically by controlled cooperative studies, which have lead to an unexpected degree of standardization in pediatric nephrology. The Arbeitsgemeinschaft für Pädiatrische Nephrologie (APN) is conducting its own cooperative studies in close contact with the International Study of Kidney Diseases in Children (ISKDC). In Steroid Study II of APN two regimens of initial prednisone treatment are compared: standard initial versus short initial prednisone therapy. Preliminary results reveal that a complete remission of NS can be obtained with less than half of the prednisone dose usually applied by the standard regimen. The relapse rate after initial treatment, however, seems to be the same in both groups. In the Cytotoxic Drug Study I of APN the effectiveness of chlorambucil (0.15 mg/kg for 8 weeks) or cyclophosphamide (2 mg/kg for 8 weeks) in the treatment of steroid toxic frequent relapsers without steroid dependency (FRNS) and with steroid dependency (SDNS) are compared. In FRNS the treatment is followed by long lasting remissions in most cases, while in SDNS relapses occur very soon again. The latter group, therefore, does not profit from cytotoxic drugs in the dosage used. In a new trial, therefore, it is asked whether patients with SDNS will profit more from prolongation of cyclophosphamide treatment for 12 weeks.