Plasma electrolyte concentrations in premature infants are unstable, and hyperkalemia may induce significant, even life-threatening, symptoms in tiny infants. The medical records of 95 premature infants were retrospectively reviewed. Patients with a major congenital anomaly or mortality within 24 h after birth were excluded. Plasma electrolytes, blood urea nitrogen, creatinine, blood pH, urine output, and related clinical conditions during first 96 h of life were analyzed. Plasma potassium concentrations had significant negative correlations with gestational age and birth weight (p < 0.05). Infants with a gestational age of less than 29 weeks had significantly higher potassium concentrations (average 5.9 +/- 0.3 mEq/L, peak 7.8 +/- 0.4 mEq/L, p < 0.05) than other gestational age groups, and their plasma potassium levels were significantly higher at 24 and 48 h of age (p < 0.05). Forty-two infants (42/95, 44%) had peak plasma potassium concentrations greater than or equal to 6 mEq/L. With statistical analysis, the hyperkalemic infants comprised of significantly (p < 0.05) fewer males (31% vs. 55%), they had more-severe respiratory distress syndrome (RDS) (grades 2 +/- 0 vs. 1 +/- 0, p < 0.05), and needed more frequent use of inotropics (52% vs. 23%, p < 0.05) compared to normokalemic infants. In conclusion, hyperkalemia during the first 2 days of life is common in extremely premature infants. Small gestational age, very low birth weight, female gender, high RDS grade, need of exogenous surfactant and inotropic agents, delayed feeding, and a high mortality rate were observed in hyperkalemic infants.