Magnesium sulfate (MgSO4) has been successfully used to inhibit premature labor. A retrospective review was performed on the use of MgSO4 as a tocolytic agent at Memorial Hospital, Long Beach, California, during a 4-year period (1978-1982). Three hundred fifty-five patients with diagnoses of premature labor were treated with MgSO4 after transport from another hospital. Two hundred seventy-four patients (77%) had a singleton pregnancy with intact membranes, 38 (11%) had a singleton pregnancy with ruptured membranes, 35 (10%) had a multiple gestation with intact membranes, and eight (2%) had a multiple gestation with ruptured membranes. Delivery was successfully delayed in the majority of patients, and the incidence of unexplained failure of tocolysis was only 2%. Side effects occurred in 24 patients (7%) and necessitated stopping the drug in only seven (2%). Serum magnesium levels are reported and the use of MgSO4 in patients with significant vaginal bleeding is discussed. MgSO4 was found to be a successful, inexpensive, and relatively nontoxic tocolytic agent that had few side effects.