PURPOSE: The aim of this study was to characterize the changes in the incidence and clinical characteristics of nonoliguric hyperkalemia (NOHK), together with plasma potassium levels, according to the fluid therapy strategies for extremely low birth weight infants (ELBWIs) during the first few days of life. METHODS: This retrospective study enrolled ELBWIs. We analyzed the occurrence of NOHK, plasma potassium levels, other biochemical data, and fluid balances according to historically controlled strategies such as conventional limited-volume supply and low-dose calcium supplementation (P1), increased-volume supply and high-dose calcium supplementation (P2), and early aggressive nutrition (EAN) and high-dose calcium supplementation (P3). RESULTS: The incidence of NOHK and the plasma potassium levels in P2 (127 ELBWIs) were not different from those in P1 (39 ELBWIs). However, arrhythmia and fatality significantly decreased in P2 compared to those in P1. In P3 (68 ELBWIs), the incidence of NOHK after 24 h and the plasma potassium levels after 36 h of life were significantly reduced compared to those in P1 and P2. Neither arrhythmia nor fatality developed in P3. CONCLUSION: EAN combined with high-dose calcium supplementation could be a potential strategy for the prevention of NOHK along with consequent arrhythmia and fatality in ELBWIs.