This report concerns two boys with minimal change nephrotic syndrome progressed to renal failure. The first case aged 17 being a steroid sensitive infrequent relapse developed acute renal failure at his third relapse and recovered soon after the treatment with diuretics and corticosteroids. The second case aged 15 being a steroid dependent frequent relapse became steroid resistant at his 11th relapse and progressed to renal failure seven months later. As the causes of renal failure, acute tubular necrosis and tubular obstruction by casts were suspected in the former. Renal vein thrombosis, morphological transition of renal histology, hemodynamic change and change in glomerular permeability might be occurred in the latter. Renal failure is a rare complication of minimal change nephrotic syndrome and the cause is variable. Precise diagnosis and prompt treatment should be needed to improve the prognosis.