This study analyzes the effects of the Oklahoma City bombing and 9/11 terrorist attacks on stress, smoking, and smoking quit attempts using 1,657,985 observations from the Behavioral Risk Factor Surveillance System. Regression discontinuity results suggest that, in the fourth quarter of 2001, stress increased by nearly an extra half day per 30 days (11.9%) among ever smoking adults. In the 2 years after 9/11, smoking prevalence increased by 1.1 percentage points (2.3%) among ever smoking adults, resulting in between 950,000 and 1,300,000 adult former smokers becoming smokers again because of terrorism. The net cost to the government was between $530 million and $830 million through the end of 2003. Adults reported disproportionate stress increases based on community military participation and education. Simultaneity between smoking and stress is addressed by an instrumental variables model, providing validity to the hypothesized causal pathway between terrorism, stress, and smoking. This model suggests that an extra day of stress per 30 days causes a 3.4 percentage point increase in smoking among ever smoking adults. Results help to quantify a hidden cost of terrorism and provide a better understanding of utility maximization during periods of high stress. (JEL I12, I18)