We retrospectively examined the usefulness of antibiotic therapy for management of first-trimester threatened abortion in women with previous spontaneous abortion. From 1993 through 1999, women with first-trimester threatened abortion received antibiotic therapy. Only those with gestational age less than 9 weeks and previous spontaneous abortion were included in this analysis. Women with mild abdominal cramping received amoxicillin plus erythromycin for 1 week; those with severe abdominal pain received amoxicillin plus clindamycin for 1 week. Recurrence was documented on the basis of either lower abdominal pain or vaginal bleeding. Of the 23 women included, 15 (65%) had abnormal vaginal flora (a score above 4, Nugent's criteria). Seven of 16 women who received amoxicillin plus clindamycin and three of seven who received amoxicillin plus erythromycin had complete resolution of lower abdominal pain and vaginal bleeding without recurrence (p=1). The recurrence rate was higher, though not significantly, in women with abnormal bacterial vaginal flora (8/15 vs. 2/8, p=0.379). Twenty-two (96%) of the 23 pregnancies were carried to term, with no identifiable neonatal anomalies. These results suggest the usefulness of early antibiotic therapy in preventing pregnancy loss in women with threatened abortion early in the first trimester, and warrant further clinical trials.