Pathogenesis and optimal treatment and prevention of preterm labor remain incompletely understood. Entry of cervical/vaginal microorganisms into lower uterine tissues has been implicated in preterm labor and may be amenable to specific therapy. Fifty-eight women with less than 34 completed weeks of gestation and without other obstetric complications, who were receiving intravenous tocolytics because of uterine contractions and who had cervical alteration (less than 5 cm dilated), were enrolled in a prospective randomized, double-blinded evaluation of 7 days of adjunctive therapy with enteric-coated erythromycin base (333 mg three times daily by mouth) versus placebo. Microbiologic examination included cultures for Neisseria gonorrhoeae, Chlamydia trachomatis, and group B streptococcus. Fifty-eight women with singleton pregnancies (29 erythromycin; 29 placebo) completed the protocol. Among women with cervical dilatation greater than or equal to 1 cm at the beginning of treatment, mean time until delivery was 32.5 days with erythromycin and 22.4 days with placebo treatment (p = 0.027). Of the erythromycin-treated women, seven of eight were delivered at greater than or equal to 37 weeks and only three of nine placebo-treated women were delivered at greater than or equal to 37 weeks (p = 0.035). Orally administered enteric-coated erythromycin as adjunctive treatment of pregnant women in labor less than or equal to 34 weeks is well tolerated. Adjunctive erythromycin given to women treated for preterm labor less than or equal to 34 weeks is associated with prolongation of pregnancy and delivery at 37 weeks only in women with cervical dilatation at the beginning of treatment.