BACKGROUND: Pilot studies suggest that a single, 2-g oral dose of azithromycin may be an alternative to a 2.4-MU intramuscular dose of penicillin G benzathine in the prevention and treatment of syphilis. We evaluated the efficacy of treatment with azithromycin in a developing country. METHODS: A total of 328 subjects, 25 with primary and 303 with high-titer (a titer of at least 1:8 on a rapid plasmin reagin [RPR] test) latent syphilis, were recruited through screening of high-risk populations in Mbeya, Tanzania, and randomly assigned to receive 2 g of azithromycin orally (163 subjects) or 2.4 million units of penicillin G benzathine intramuscularly (165 subjects). The primary outcome was treatment efficacy, with cure defined serologically (a decline in the RPR titer of at least two dilutions by nine months after treatment) and, in primary syphilis, by epithelialization of ulcers within one or two weeks. RESULTS: The average age of participants was 27.0 years, 235 (71.6 percent) were female, and 171 (52.1 percent) were seropositive for human immunodeficiency virus. Cure rates were 97.7 percent (95 percent confidence interval, 94.0 to 99.4) in the azithromycin group and 95.0 percent (95 percent confidence interval, 90.6 to 97.8) in the penicillin G benzathine group (95 percent confidence interval for the difference, -1.7 to 7.1 percent), achieving prespecified criteria for equivalence. Cure rates were also similar three and six months after treatment in the two groups and in all subgroups. Cure rates at three months were 59.4 percent (95 percent confidence interval, 51.8 to 67.1) in the azithromycin group and 59.5 percent (95 percent confidence interval, 51.8 to 67.3) in the penicillin G benzathine group and at six months were 85.5 percent (95 percent confidence interval, 79.4 to 90.6) and 81.5 percent (95 percent confidence interval, 74.8 to 87.4), respectively. CONCLUSIONS: Single-dose oral azithromycin is effective in treating syphilis and may be particularly useful in developing countries in which the use of penicillin G benzathine injections is problematic. However, recent reports of azithromycin-resistant Treponema pallidum in the United States indicate the importance of continued monitoring for resistance.