PURPOSE: The purpose of the study was to evaluate serum magnesium(Mg) concentration in very low birth weight(VLBW) infants during the first three weeks of life and to assess its relation to diseases of prematurity. METHODS: We measured serum Mg level at 0, 1, 2, and 3weeks of life in VLBW infants and analyzed its correlations with diseases of prematurity. Ninety-five VLBW infants(mean gestational age 30.4wks, birth wt 1304gm) who survived 30days were selected. Seven infants who had been treated with magnesium sulfate prenatally were excluded. RESULTS: Serum Mg level decreased linearly during the first three weeks of life(P=0.03). Serum Mg level at birth had no significant relation to gestational age, birth weight and serum calcium concentration. Serum Mg level at birth were higher within normal range in infants with respiratory distress syndrome(n=20, 2.6mg/dl vs. n=68, 2.2mg/dl, P=0.036), patent ductus arteriosus (n= 19, 2.5mg/dl vs. n=69, 2.2mg/dl, P=0.035) and bronchopulmonary dysplasia(BPD)(n=15, 2.6mg/dl. vs. n=73, 2.2mg/dl, P=0.01) than in infants without them. Serum Mg level at first week of life were similar(2.3mg/dl. vs. 2.2mg/dl, P=0.51) and serum Mg level at second and third weeks of life were significantly lower in infants with BPD than in control(1.9mg/dl vs. 2.2mg/dl, P=0.002 and 1.6mg/dl vs. 2.2mg/dl, P=0.001, respectively). CONCLUSION: Serum Mg level during the first three weeks of life decreased linearly. Serum Mg level of infants with BPD at birth was higher within normal variation than in infants without BPD. And serum Mg level of infants with BPD at second and third weeks of life were lower than control. Thus, we suggest that Mg deficiency during the first three weeks of life might play a role in the pathogenesis of BPD.