Background. Magnesium sulfate continues to be widely used as a tocolytic agent despite a paucity of evidence supporting its use. Many practitioners use prolonged courses of magnesium sulfate, sometimes for months. This study was conducted to determine maternal and neonatal outcome of patients exposed to prolonged tocolytic magnesium sulfate. Methods. A retrospective review of maternal and neonatal charts (1995–2003) of pregnancies that received tocolytic magnesium sulfate. Cases who received magnesium sulfate >48 h (n=78) were compared to controls who received it ≤48 h (n=77) for maternal side effects and neonatal outcome. Results. Cases were more likely to be nulliparous (55.1% versus 37.7%, p=0.044), multiple gestations (33.3% versus 10.4%, p=0.001), and of lower gestational age (29.0±3.9 versus 30.5±3.8 weeks, p=0.017) compared to controls. The median magnesium sulfate infused was 154 (78–5,500) versus 54 (8–86) g (p<0.001) and the highest maternal magnesium level was 6.5±1.7 versus 5.6±1.9 mg/dl (p=0.002) in cases and controls, respectively. Cases were more likely to have ≥1 adverse side effect (30.8% versus 15.6%, p=0.045). The median neonatal magnesium level was significantly higher in cases (3.3 (1.4–7.2) versus 2.6 (1.1–5.2) mg/dl, p=0.016); however, neonatal mortality and other neonatal morbidity rates were similar in both groups. Abnormal bone mineralization was encountered in 3 neonates (cases). Conclusions. Maternal morbidity rate is higher with prolonged intake of tocolytic magnesium sulfate compared to ≤48-h regimen. Despite similar neonatal morbidity and mortality rates, bone demineralization in the neonates may be encountered.