Obesity may affect the respiratory health of people with asthma. Because the temporal trends in the prevalence of obesity among people with asthma have not been described in the United States, our objective was to describe these trends. Using data from National Health and Nutrition Examination Survey (NHANES) I (1971-1975), II (1976-1980), and III (1988-1994), the authors examined changes in the prevalence of obesity during the period covered by these surveys. The age-adjusted prevalence of current asthma was 3.5% for NHANES I, 3.1% for NHANES II, and 5.2% in NHANES III. Among people with current asthma, age-adjusted mean body mass index increased from 26.1 kg/m2 in the NHANES I to 28.0 kg/m2 in NHANES III, and the age-adjusted prevalence of obesity increased from 21.3 to 32.8%. Among people without asthma, age-adjusted mean body mass index increased from 25.4 kg/m2 in NHANES I to 26.6 kg/m2 in NHANES III, and the prevalence of obesity increased from 14.6 to 22.8%. These results show that people with asthma are far more likely to be obese than people who do not have asthma. Because excess weight may adversely affect the respiratory health of people with asthma, weight management for overweight and obese patients with asthma may be an important component in the medical care of these patients.