The aim of our study was to characterize renal function and its relationship to blood pressure in healthy young Caucasian men born with a birth weight under 2,500 g (LBW). Urinary protein patterns, N-acetylglucosamine and gamma-glutamyltransferase activities, fractional sodium and potassium excretions, glomerular filtration rate, blood pressure, and erythrocyte Na+/K+-ATPase activities were determined in 65 subjects, of whom 49 were born with LBW. Signs of glomerular or tubular damage were not detected in the LBW population. However, the blood pressure and the renal sodium excretion were inversely correlated to the subjects' birth weight and were higher in LBW subjects than in controls. In contrast, the erythrocyte Na+/K+-ATPase activities were lower in LBW subjects. An inverse correlation was detected between the subjects' Na+/K+-ATPase activities and the renal sodium excretion or blood pressure. In summary, our results suggest that: (1) in young LBW Caucasian males signs of early glomerular and tubular impairment are not present; (2) the elevated renal sodium excretion may be a result of higher blood pressure; (3) the alteration of Na+/K+-ATPase activity might play a role either in the elevation of blood pressure and/or in the enhanced natriuresis of LBW subjects.