We introduced minimal necessary criteria and methods for noninvasive laboratory diagnosis and follow-up of functional proteinuria in youths: stress tolerance test, determination of total urinary proteins (TP) and their separation and identification with gradient SDS-PAGE. Renal functional adaptability in conditions of complete rest, in routine daily activity and after several hours of active physical effort has been evaluated by the tolerance test. Excretory urinary proteins, as the most appropriate markers, were analyzed with noninvasive methods. Excretory TP demonstrated the quantity and dynamics of the proteins excreted during the tolerance test. Separation with gradient (4-22.5%) SDS-PAGE provided differentiation of functional proteinuria from the other orthostatisms, through the protein fractions present and the constant finding of apolipoprotein AI. The investigation comprised 19 youths with orthostatism and 20 healthy subjects without orthostatism, all between the age of 10 and 18 years. The subjects without orthostatism excreted proteins in normal limits during the stress tolerance test. SDS-PAGE of the urinary samples, obtained during the tolerance test, in five subjects significantly differed from that in the other 14 subjects. According to the protein fractions present as well as from the dynamics of the protein excretion, orthostatism in these five subjects probably originated from organ structural lesions. In the other 14 subjects functional proteinuria was present, but there were differences among them in the dynamics of the appearance and disappearance of the orthostatism, which has been used for arbitrary division of the orthostatic and the so-called march-proteinuria.