Smoking generally suppresses body weight below "normal," and smoking cessation allows weight to return to normal. This weight gain following cessation appears to be due to a transient increase in eating coupled with the removal of acute metabolic effects of each cigarette, with no change in physical activity. Nevertheless, tobacco smoke (and specifically nicotine) does not appear to be simply either an anorectic or a thermogenic agent. Although there may be no easy explanation for the effects of smoking on energy balance, the most parsimonious explanation may be that smoking lowers body weight "set point" and cessation raises set point. The transient changes in eating are therefore secondary to the changes in body weight set point. This notion is supported by animal research with nicotine as well as with other drugs, and it is also supported less directly by the pattern of changes observed with changes in smoking status among humans. A set-point explanation for weight gain after smoking cessation may also help explain the lack of success of interventions designed to prevent this weight gain.