There are few reported studies of water balance and electrolyte abnormalities in extremely low birthweight (ELBW) infants weighing < 1000 g nursed in high humidity. We retrospectively analyzed water balance, electrolyte and acid-base balance in 100 ELBW infants, of whom 72 were appropriate for gestational age (AGA) and 28 were small for gestational age (SGA). They were cared for in closed incubators at almost full ambient humidity. Fluid intake was restricted to 50-60 mL/kg on the first day of life and was adjusted to maintain normal serum Na concentration. Weight loss in AGA and SGA infants was 21.6 and 16.7%, respectively, and was associated with large urine volume rather than insensible water loss. The incidence of hypernatremia (> 150 mEq/L) and hyponatremia (< 130 mEq/L) was 8.0 and 33.3%, respectively. The incidence of hyperkalemia in AGA infants was 37.0%, and 14.8% in the SGA group. However, hyperkalemia requiring treatment was rare. The incidence of late metabolic acidosis in AGA and SGA infants was 84.6 and 37.5%, respectively (P < 0.01). The difference in water balance and electrolyte abnormalities in AGA and SGA infants needs to be taken into account in managing ELBW infants.