To evaluate the development of renal function in fetal malnutrition conditions, we tested glomerular filtration rate and fractional sodium excretion (FENa) of premature twins at birth (less than or equal to 35 weeks gestational age) with birthweight discordance, divided according to birthweight (less than or equal to 10% group A; higher birthweight group B). Groups A and B of twins are comparable in fluid intake, biochemical serum tests (glucose, sodium, potassium, albumin, total protein, osmolality and hematocrit) and also in percent weight decrease at 24 h of life. Moreover, both creatinine clearance (CrCl) and FENa result respectively correlated directly (r = +0.63) and indirectly (r = -0.46) with gestational age of twins. Consequently, CrCl (8.3 +/- 4.3 vs 8.3 +/- 4.5 ml/min/1.73 m2) and FENa (3 +/- 2.8 vs. 2.7 +/- 2.8%) do not result different in relation to birthweight discordance of premature twins. In conclusion, at birth, gestational age is the main determinant of kidney function in premature twins with birthweight discordance.