Objective: To examine the influence of positive affect and mood-management in the completion and success of an Internet-based smoking cessation intervention. Methods: Participants were recruited online (n = 1000) and randomized to 4 different interventions. Half of the participants received a mood-management tool. Results: Retention was predicted by positive affect at 3 and 12 months. There was a higher 12-month abstinence rate among those who were assigned to the mood-management intervention and had initial low positive affect. Conclusions: The study supports the inclusion of mood-management tools in smoking cessation interventions and indicates that positive affect increases persistence to quit smoking.