Azithromycin has in vitro activity against Treponema pallidum and is effective against experimental syphilis in rabbits. We undertook an open, noncomparative pilot study of oral azithromycin (500 mg once daily for 10 days) to treat 16 patients with primary or secondary syphilis who were seronegative for human immunodeficiency virus. Cure was documented for 11 of 13 patients observed ⩾3 months; three patients were lost to follow-up. The serological response of one patient with secondary syphilis was indeterminate, and one patient with primary syphilis had either relapse or reinfection. Four patients had mild gastrointestinal side effects, and another patient had an episode of nausea and vomiting; all side effects occurred in the first 3 days and resolved spontaneously as treatment continued. Azithromycin shows promise as an alternative agent for treatment of early syphilis; controlled trials and assessment of other dosage regimens are indicated.