In infancy, the kidney by itself is immature and the body fluid composition is extremely different from that in adults. Further more, there are various renal diseases in children with characteristics different from those in adults. Thus in childhood, consideration for aging is critical for evaluation of renal function, and diagnosis of the disease. Recently, mass screening for infants or children in kindergarten or school has enabled us to find many children with renal disease, in whom symptoms are not overt. Thus the disease can be treated in an early phase. This may give a better prognosis for such children. To make an early diagnosis of renal disease it is essential to consider what laboratory tests are chosen and how the data accumulated are interpreted. In the present study, individual laboratory tests for the diagnosis of renal diseases and an indication of such tests as well as interpretation of the data are reviewed.