Renal function in the VLBW is substantially immature relative to the term and more mature preterm neonates leading to significant fluid, electrolyte and metabolic derangements. In order to determine the magnitude of these problems in the VLBW, the clinical course in the immediate neonatal period (first 4 wks of life) of all 145 preterm neonates admitted to our neonatal intensive care unit with birth weights < 1,000 g (mean ± SEM: 755±157g, range: 280-1,000g and mean gestational age: 26.2±2.2 wks, range: 22.4-32.7 wks) were reviewed. All babies had multiple serum electrolyte measurements and all were weighed thrice per day. Daily fluid, electrolyte and total parenteral nutrition were adjusted at least once or twice per day. 44/145 babies (30.3%) had at least 1 episode of hyperkalemia (arterial or venous K>7.0 mmol/L), 89/145 (61.4%) had hyponatremia (Na<130 mmol/L) and 34/145 (23.4%) had hypernatremia (Na>150 mmol/L). 106/145 (73.1%) had at least one episode of hyperglycemia (serum glucose >8.0 mmol/L). 49/145(33.8%) required insulin for the hyperglycemia except one who had insulin for hyperkalemia. 24 babies (16.6%) had renal failure (urine flow rate <0.5 ml/kg/h) with the mean onset at 15.6±12.6 days. Maximum blood urea nitrogen was 14.7±4.6 mmol/L (range: 4.3-26.4 mmol/L) and maximum serum creatinine was 242±73 μmol/L (range 126-405 μmol/L). Drugs used in these babies included furosemide, ethacrynic acid and indomethacin. Data show that significant metabolic and electrolyte derangements occur commonly in VLBW neonates underlining the need for close surveillance and monitoring and further studies on renal function in this population.