To identify predictors of attempts to stop smoking and predictors of relapse. This study included 2431 smokers from pre-existing Internet panels in the United States, United Kingdom, Canada, France, and Spain. These panel members are Internet users who have registered voluntarily and agreed to participate in various online research studies. Respondents were aged 35-65 years, smoked >or= five cigarettes per day and intended to stop smoking in the next 3 months. They were followed every 3 months for up to 18 months via Internet contact on measures relating to quit attempts, smoking status, motivation to quit, nicotine cue, weight and weight concern, health-related factors, withdrawal symptoms, and smoking cessation aids. In this study, recent quit attempts strongly predicted future attempts, but also predicted subsequent relapse. Motivation to quit was predictive of future attempts but not of relapse/abstinence following the attempts. Relapse to smoking was associated with nicotine dependence, exposure to smoking cues, craving, withdrawal symptoms, and lack of smoking cessation aids. The findings lend support to a model of cessation in which level of motivation to stop generates quit attempts but plays little role in relapse. Dependence, social smoking cues, and a recently failed quit attempt are important factors in relapse.