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Renal biopsy, the introduction of immunohistologic methods and electron microscopy have allowed the differentiation of clinicopathologic entities associated to nephrotic syndrome. Two main categories must be differentiated: in the first, diffuse lesions of the glomerulus, including those secondary to specific diseases the same as those that are apparently primary, are responsible for the increased permeability of glomerular capillaries. Any one of the following clinical signs suggests this category: acute onset with nephritic syndrome, moderate nephritic syndrome, moderate nephrotic syndrome, gross hematuria, persistent hypertension and/or renal failure, poor selectivity of proteinuria and drop in complement serum levels (C3). In the second category, known as idiopathic nephrotic syndrome, the mechanism of disorder of the glomerular capillary is unknown and the nephrotic syndrome is more marked. In most cases with idiopathic nephrotic syndrome, minimal glomerular lesions (MGL) are present. The clinicopathologic correlation among these three types of lesions shows that the type with MGL is characterized by selective proteinuria, absence of hematuria, good response to corticosteroids and good outlook; whereas in types with diffuse mesangial proliferation (DMP) and segmentary sclerosis, proteinuria is frequently non selective, hematuria shows in 50 to 75% of the patients; prognosis is poor. However, MGL, DMP and focal segmentary glomerular sclerosis are not different entities, but represent variants of the same disease. Considering that corticosensitive nephrosis to this moment is the most common cause of the nephrotic syndrome, especially in children under 8 years, renal biopsy should be done only under two circumstances: a) when the clinical symptoms suggest diffuse glomerular lesions and b), when resistance to corticosteroids becomes evident.