OBJECTIVE To evaluate the efficacy of azithromycin in preventing congenital syphilis. METHOD Five pregnant women with syphilis who were allergic to penicillin were given azithromycin, 1 g daily orally or intravenously, in different hospitals. The duration of the therapy ranged from 1 day to 10 days. A second course of therapy was provided at 28 weeks gestation. The babies were given a physical examination and blood test for serum rapid plasma reagin test (RPR), treponema pallidum hemagglutination test (TPHA), and fluorescent treponemal antibody adsorption test (FTA-ABS-19-sIgM) within three months after birth. RESULTS Five infants born to these mothers developed skin rashes. Four of the infants had hepatomegaly and one showed osteochondritis. The tests RPR, TPHA, and FTA-ABS-19-sIgM were positive. The RPR titers varied from 1:64 to 1:256 and the babies were diagnosed with congenital syphilis. They were successfully treated with penicillin. CONCLUSIONS Successful therapy for syphilis during pregnancy demands maternal care as well as prevention or cure of congenital infection. The failure of azithromycin in preventing congenital syphilis in our report suggests that azithromycin should not be recommended as an alternative in treating syphilitic pregnant women or fetal syphilis.