The finding of proteinuria in the pediatric age is a frequent phenomenon. Once detected, the immediate goal is to investigate other evidences of renal disease or its participation in systemic diseases. If proteinuria is an isolated finding, its transitoriness, persistence or relation with posture must be determined. Transitory proteinuria is a mild finding with good outlook; however, in patients with orthostatic or persistent asymptomatic proteinuria, the prognosis is uncertain; however, the measures taken must be conservative without restriction of physical activity and only in case of association to other signs, or finding of any change in renal function tests, the possibility of renal biopsy should be evaluated. The presence of proteinuria above 1 g/l. in a patient with acute nephritic syndrome, may be a sign of poor prognosis if it persists over one month and is indicative of renal biopsy. If this same degree of proteinuria is found in a nephrotic syndrome without hematuria or arterial hypertension, the necessity to practice renal biopsy is not indicated, except in the case it does not disappear following adequate treatment with corticosteroids.