BACKGROUND: Early fluid and electrolyte imbalances may be associated with an increased risk of bronchopulmonary dysplasia. OBJECTIVE: We sought to establish an association between fluid and electrolyte balance in the first week of life and the risk of bronchopulmonary dysplasia. METHODS: Clinical charts of 205 neonates <32 weeks gestational age and/or <1,250 g birth weight (admitted to our NICU between 1997 and 2008) were analyzed. Clinical features, fluid and electrolyte balance were analyzed for the first 7 days of life using multivariate models of generalized estimation equations. A p value <0.05 was considered significant in all of the hypothesis tests. RESULTS: The prevalence of bronchopulmonary dysplasia was 22%. Lower gestational age and birth weight, male gender, less frequent use of antenatal steroids, respiratory distress syndrome, use of surfactant, patent ductus arteriosus, duration of invasive ventilation and NICU stay were significantly associated with bronchopulmonary dysplasia. The variation in serum values of potassium, phosphorus and creatinine during the first week of life also revealed an association with bronchopulmonary dysplasia. Higher mean plasma calcium values were associated with spontaneous closure of the patent ductus arteriosus. The use of indomethacin to induce patent ductus arteriosus closure was significantly higher in bronchopulmonary dysplasia patients. CONCLUSIONS: Differences in renal function and tubular handling of potassium and phosphorus are present during the first week of life among preterm neonates who will develop bronchopulmonary dysplasia. The higher rate of patent ductus arteriosus and indomethacin use may influence these differences. Serum levels of calcium also appear to play a role in spontaneous ductus arteriosus closure.