In a multicenter series of randomized prospective double-blind controlled studies, ritodrine hydrochloride was compared with either ethanol or placebo in treatment of idiopathic preterm labor. When compared with controls, there was, among offspring of ritodrine-treated mothers, a significantly reduced incidence of neonatal death and respiratory distress syndrome (P less than .05 in both comparisons) and a significantly higher proportion of infants achieving 36 weeks' gestation (P less than .05) or birth weight greater than 2500 g (P less than .05). There was also a significant improvement in gestational age at delivery (P less than .05) and in the number of days gained in utero (P less than .001) among ritodrine-treated patients as compared with controls. These results, coupled with a finding of generally acceptable side effects, have contributed to ritodrine's becoming the first drug approved for the treatment of preterm labor in the United States.