Infants born preterm and of low birth weight are known to be at increased risk for early onset of cardiovascular and renal disease in adult life. This has been related to low nephron mass due to inadequate or early termination of glomerulogenesis in utero and during the perinatal period. Risks for subsequent development of hypertension and kidney disease include proteinuria, excessive weight gain during early life with insulin resistance and supplemental high calorie feedings. The long-term goal is for early diagnosis of those infants who are at risk for future development of hypertension and kidney disease so that the investigators might intervene to potentially avert progression to adult disease. The objective of this clinical trial is to acquire data on the natural history of neonatal kidney function and size in infants born preterm during the first 2 years of life. This will be done through the use of standard serum and urine markers as well as non-invasive ultrasound technology. The central hypothesis of this clinical trial is that a subgroup of patients born preterm and of low birth weight will demonstrate early markers of kidney injury including elevated serum cystatin C, proteinuria and low kidney size. This hypothesis has been formulated on the basis of preliminary data from our group studying this question retrospectively in older children born prematurely who have developed overt kidney disease. The rationale for the proposed research is to develop early serum and demographic markers of pre-clinical kidney disease so that early intervention can occur. The proposed clinical trial is innovative because it will investigate the risk factors for kidney dysfunction at a pre-clinical stage with the idea of gaining more knowledge regarding therapeutic interventions. In addition, the study will assess serum cystatin C as a surrogate test for glomerular filtration rate which could indicate worsening kidney function at an earlier stage than serum creatinine. The proposed research is significant because it is expected to identify at-risk patients for future renal impairment and to prospectively monitor the persistence of proteinuria and its effect on kidney function in the short term.