With the introduction of penicillin after World War II, the incidence of syphilis in the United States decreased. Because of penicillin's great success, clinical trials stopped after an initial period of intensive investigation. Syphilis is a difficult disease to study; the natural history may span decades in an individual, and diagnosis and outcome are usually defined serologically, not clinically or bacteriologically. Although the recommended penicillin regimens changed, clinical trials were not repeated. Furthermore, because the early studies occurred before modern clinical-trial methodology was developed, interpretation of the results is difficult. As a result, while current regimens for syphilis therapy are effective, they may or may not be optimal. With the accumulation of reports of treatment failures and the recent appearance of human immunodeficiency virus, current regimens for the treatment of syphilis are being questioned. As background for a meeting at which treatment guidelines were reviewed, the available literature on syphilis therapy is summarized herein.