A few previous studies have looked at the relationship between pregnancy outcome and magnesium nutritional intake and found that magnesium supplementation which could have beneficial effects on perinatal outcome. We performed a study to examine the relationship between the levels of magnesium in drinking water and the risk of delivering a child of very low birth weight (birthweight less than 1500 g; VLBW). The study population comprised 1,781 women residing in 252 municipalities who had a first parity singleton birth between January 1, 1993 and December 31, 1997, and for whom complete information on maternal age, education, gestational age, birth weight, and sex of the baby were available. Data on magnesium levels were obtained from the Taiwan Water Supply Corporation (TWSC). The results of our study show that there is a significant trend toward a decreased risk of having a child of VLBW with increasing magnesium levels in drinking water. Only the specific reversibility of the symptoms through nutritional magnesium in drinking water supplementation might demonstrate the existence of a pathogenic link.