The objective of this study was to clarify the relationship between the blood potassium and calcium levels, and the efficacy of prophylactic calcium (Ca) administration early in life for nonoliguric hyperkalemia in extremely low birthweight (ELBW) infants. This was a retrospective study including 55 ELBW infants with gestational age less than 26 weeks (mean, 24.4 weeks; mean body weight, 681 g). The plasma potassium concentration and whole blood ionized calcium (iCa) concentration were measured every 2 to 3 hours. Laboratory data obtained up to 24 hours after birth were collected. The infants were divided into two groups based on whether or not Ca gluconate was administered prophylactically starting at admission (prophylactic and nonprophylactic group). There was a negative correlation between the plasma potassium and iCa levels at 12 and 24 hours, and the maximum plasma potassium level was higher in the hypocalcemia group (minimum iCa level, < 0.9 mmol/L) than in the normocalcemia group. The iCa level was significantly higher and the plasma potassium level was significantly lower in the prophylactic group than in the nonprophylactic group at 12 and 24 hours. The increment in the plasma potassium level at 24 hours compared with that at admission was significantly lower in the prophylactic than in the nonprophylactic group. Nonoliguric hyperkalemia may be attenuated by maintaining the iCa level within normal limits by prophylactic Ca administration early in life. Prospective studies are needed to confirm this.