The purpose of this study was to examine the associations between smoking, physical inactivity, obesity, and asthma severity among US adults. The magnitude of these associations was very strong. For example, those who visited an emergency room in the past year were 60% more likely than those who did not to smoke; those who used an inhaler > or =15 times in the past month (versus those who did not use an inhaler) were 90% more likely to be physically inactive; and those who had asthma symptoms all the time in the past 30 days (versus those with no symptoms) were 80% more likely to be obese.