The Magnesium and Neurologic Endpoints Trial was a randomized controlled trial (RCT) done to learn whether or not receiving magnesium sulfate during preterm labor could prevent cerebral palsy. Unexpectedly, in the tocolytic arms of the trial, seven (including one set of twins) of 46 cases assigned to receive magnesium ended in total pediatric mortality (fetal + neonatal + postneonatal), compared to none of 47 cases assigned to other tocolytics ending in death. The difference between the two treatment arms is highly statistically significant (risk difference 15.2%; 95% confidence interval 4.8, 25.6;P= .006). If this relationship is confirmed by experimentation with animals or through the conduct of a large RCT at other institutions, it is possible that tocolytic magnesium will be found to be associated with the deaths of several thousand newborns in the United States annually. If the true excess total pediatric mortality is 10%, and if magnesium accounts for 40% of all tocolytics used, then tocolytic magnesium increases the absolute number of infant deaths by about 4800 every year.