The objectives of the proposed study are to test whether smokers with PTSD lapse early to smoking and relapse to smoking at significantly higher rates than smokers without PTSD; and to elucidate the explanatory mediating mechanism(s) underlying the relations between PTSD and poor smoking cessation outcomes. To achieve these objectives, the proposed investigation employs a prospective design that follows a sample of 84 smokers (42 with PTSD and 42 without PTSD) for three months as they attempt to quit smoking on their own. Prior to initiating the cessation attempt, subjects will be scheduled to visit the laboratory after not smoking for a 12-hour experimental abstinence period (i.e., laboratory analogue of the early cessation attempt that is to follow). During this smoking deprivation session, subjects will complete a physical stress challenge procedure (voluntary hyperventilation). Affective processes theorized to mediate the link between PTSD and poor smoking cessation outcomes will be measured within this laboratory procedure using multi-method (subjective, physiological, and behavioral) indices. Then, one-week later, beginning on quit day and continuing at regular intervals until twelve weeks after the quit date, subjects will be prospectively evaluated within the lab to biochemically verify smoking status as well as assess smoking outcomes, affective symptoms, and withdrawal symptoms. In addition, for the initial 14 days of the cessation attempt, Ecological Momentary Assessments (EMA) will be used to prospectively measure smoking outcomes, affective symptoms, and withdrawal symptoms outside of the laboratory and within the subjects’ daily environment. The prospective laboratory assessment and EMA data will facilitate measurement of smoking cessation outcomes and the affective processes theorized to mediate the association between PTSD and those smoking outcomes.