PURPOSE:This study was designed to know the clinical significance and risk factors of hyperkalemia in very low birth weight infants (VLBWI). METHODS:We compared the incidence of hyperkalemia and its associated ECG abnormalities and mortality rate according to birth weight (102 of less than 1,000 g vs. 328 of 1,000 to 1,500 g) in 430 VLBWI who were born from Jan. 2000 to July 2004, retrospectively, and also compared the several parameter according to serum potassium (35 of hyperkalemia vs. 11 of normokalemia) in 49 selected VLBWI who need mechanical ventilation for respiratory distress without oliguria within 72 hours of life to know the risk factors of hyperkalemia. RESULTS:Overall incidence of hyperkalemia in VLBWI was 21.1%, and was significantly higher in below 1,000 g than in 1,000 to 1,500 g (32.4% vs. 17.7%, P<0.05). Hyperkalemia associated ECG abnormalities, life-threatening cardiac arrhythmia, and death were also significantly higher in below 1,000 g. There were no significant differences in maternal or infant's characteristics, laboratory findings and clinical conditions between hyperkalemic vs. normokalemic group. The incidence of cardiac arrhythmia and mortality rate were also significantly higher in hyperkalemic group. CONCLUSION:Early onset nonoliguric hyperkalemia is often associated with life- threatening cardiac arrhythmia and death in VLBWI, especially less than 1,000 g. Although perinatal risk factors were not found in this study, prospective study is needed to establish the preventive strategy and to improve the outcome.