A teachable moment (TM) refers to life events or transitions, such as physician visits, early detection screening feedback, pregnancy, and hospitalization for, and diagnosis of, disease, that inspire health behavior change. Despite strong intuitive appeal of the TM concept and over 150 studies of interventions implemented in a TM, there has been little investigation of the influence of TMs for smoking, particularly for promoting smoking cessation among those indirectly affected by a TM (e.g., relatives of cancer patients). This prospective observational study using a matched-group design will assess whether a lung cancer diagnosis increases enrollment in a smoking cessation program and cessation, and reduces smoking, among the patient's relatives/spouse who smoke. We will compare enrollment in a cessation program and cessation and smoking rates in 2 groups of smokers: 1) relatives/spouses of newly-diagnosed lung cancer patients, and 2) relatives/spouses of patients with an orthopedic condition. We will also explore mediators and moderators of a lung cancer diagnosis as a TM. This study could clarify the TM concept for smoking cessation, shed light on the underlying mechanisms of TMs, and guide future development and implementation of TM-based smoking cessation interventions.