Seventy patients with preterm labor and intact membranes were initially treated with ritodrine hydrochloride to delay preterm delivery. Tocolysis beyond 24 hours was achieved in 59 patients. Fifty-five of the 59 patients were then placed on either oral ritodrine or placebo as maintenance therapy in a randomized double-blind manner. If preterm labor recurred, the sequence of intramuscular and then oral treatment was repeated. The number of days gained after initiation of intramuscular treatment was similar in both groups (oral ritodrine = 34 days, oral placebo = 36 days). In those 55 patients receiving oral treatment, there was a smaller number of relapses requiring repeat intramuscular treatment in the oral ritodrine group (1.11 in the ritodrine patient vs. 2.71 in the placebo patient, p less than 0.05), and the mean interval between beginning oral treatment and the first relapse/delivery was 5.8 days in the oral placebo group and 25.9 in those receiving oral ritodrine (p less than 0.05). Cardiovascular side effects, notably maternal tachycardia and palpitations were frequent but well tolerated. The results suggest that oral ritodrine maintenance will decrease the incidence of recurrent preterm labor in patients who have had initial successful tocolysis.