This study was undertaken to test the effects of erythromycin as an adjunct to tocolysis for preterm labor in women with vaginal cultures positive for Ureaplasma urealyticum. The study group consisted of 18 women in active preterm labor with pregnancies between 26 and 34 weeks of gestation and intact membranes who received 500 mg erythromycin orally every 8 h for 10 days. Seventeen women with similar characteristics served as controls and received no antibiotics. In all women contractions were suppressed with ritodrine. Erythromycin treatment resulted in a statistically significant greater mean delay of delivery (36.4 days) than among the control group (23.1 days). Higher proportion of term pregnancies (7 versus 3 pregnancies), higher mean birth weight (2,745 versus 2,474 g), lower neonatal morbidity (22.2 versus 42.2%) and shorter mean neonatal hospitalization time (9.6 versus 12.1 days) were observed, although these differences were not statistically significant. Adjunctive erythromycin treatment given to women treated for preterm labor with intact membranes and positive vaginal cultures for U. urealyticum appears to prolong gestation and to improve perinatal outcome.