Many smokers and ex-smokers worry about their health. Given that worry keeps attention focused on the threat, it was expected that worrying about health in smokers would motivate them to quit and in ex-smokers may prevent relapse. Furthermore, worry was expected to influence the process of smoking cessation in interaction with self-efficacy, which is a measure of control over smoking, and with disengagement beliefs, which distorts the threatening meaning of potential motivating information. In the present study 380 smokers and 324 ex-smokers were recruited to join a prospective study with a follow-up of eight months. At T1, smoking/quitting behavior, worry and the other psychological constructs were assessed. At T2 quitting activity in smokers and relapse in ex-smokers were assessed. As expected, smokers who worried about the health effects of smoking reported higher quitting activity at T2. The three-way interactions between worry, self-efficacy and disengagement beliefs in the prospective prediction of quitting activity and relapse were significant: Among smokers with high self-efficacy combined with strong disengagement beliefs, worry led to more quitting activity. Among ex-smokers with low self-efficacy combined with strong disengagement beliefs, worry led to more relapse. The present results suggest new ways of approaching the stimulation of quitting and the prevention of relapse.