Background: The QuitCoach, an internet-based expert system for smoking cessation developed in Australia, has been publicly available since July 2003. The aims of this study were to characterise QuitCoach users, and to examine patterns of use over time, including to what extent users are engaging with the program optimally. Method: Between July 2003 and April 2006, 16,340 individuals logged on to The QuitCoach. Data were collected during a 15-minute questionnaire that all users completed in order to obtain tailored cessation advice. Results: Users were 36.4 years old (SD = 11.3); 62% were female, and 95% were currently smoking. Smokers consumed an average of 19 (SD = 9.7) cigarettes per day. Most were planning to quit in the next 30 days. Among those quit for greater than 7 days, 29% were using pharmacotherapy. Among registered users, 25% returned for a second assessment, on average after 19.4 days. Overall, 2% returned within 2 days. Those who had quit at their first assessment were more likely to return than those who were smoking (36% vs. 24%, p < 0.001). Conclusions: Most participants actually make quit attempts. Nonetheless, improved strategies are needed for encouraging repeated use. Implications: Internet-based programs have considerable potential to reach large numbers of smokers at low cost. Future plans include the development of an integrated SMS messaging component, with capacity to respond to user-initiated requests for help. A large-scale randomised trial is planned for 2008/2009.