It has been hypothesized that calcium supplementation can reduce smooth muscle contractibility and tone and that this effect can be clinically manifested by a reduction in blood pressure and a reduction in the incidence of premature delivery. Calcium supplementation has been reported to reduce blood pressure in pregnant and nonpregnant women. We performed a study to examine the relationship between the levels of calcium in drinking water and the risk of delivering a child of very low birth weight (VLBW). The study population comprised 1781 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 calcium levels were obtained from the Taiwan Water Supply Corporation. The results of our study provide evidence that there is significant protective effect of calcium intake from drinking water on the risk of delivering a VLBW baby. This is an important finding for the Taiwan water industry and human health.