BACKGROUND: The Internet has opened up new possibilities in public health, yet to date, there are few examples of online health interventions that go beyond information provision. This paper describes the users and return rates of an effective intervention for smoking cessation that has gone online. METHODS: Open enrollment of smokers via links and search engines with voluntary follow-up for an Internet-based expert system in French and English. RESULTS: Between June 1998 and March 2001, 18,361 people from 112 countries used the program. When compared to the distribution of smokers in the general population in seven main countries, smokers in the action stage (11%) were overrepresented among online program users. Among the 3591 returnees in the study period (20%), we observed a median time to return of 132.5 days. One of the strongest predictors of return was stage of change with smokers in the action stage being most likely to return (28%) and those in the precontemplation stage least likely (14%). CONCLUSIONS: The reach of an online public health intervention can be impressive in geographical and numerical terms. Although the quality of behavioral data collected was high, the utility of this medium for science should be refined.