Obesity appears more strongly associated with asthma in women than in men. It is hypothesized that a stronger linkage of obesity with nonatopic asthma than with atopic asthma may explain the sex difference. That being the case, we might hypothesize a stronger association between obesity and asthma in nonatopic individuals than in atopic ones. In this analysis, we examined the association between obesity and asthma in atopic and nonatopic people separately. A total of 1997 residents aged 18 to 79 years who participated in the 2003-2004 Humboldt study were included in the analysis. Body mass index (BMI) and waist circumference were objectively measured. Allergy skin tests were conducted to determine atopic sensitization. Respiratory allergy and physician diagnosed asthma were self-reported. Overall, 8% reported having asthma, 30% had atopic sensitization as determined by allergy skin tests, 31% reported a history of respiratory allergy, and 35% were obese defined as BMI equal to or larger than 30 kg/m(2). Compared to those with a BMI <25 kg/m(2), the odds ratio for asthma for the nonatopic subjects of those with a BMI of at least 30.0 kg/m(2) was 2.01 (95% confidence interval [CI]: 1.13, 3.59] after adjustment for sex and age. The association between obesity and asthma was not statistically significant in atopic subjects. The adjusted odds ratios for obesity versus normal weight were 2.56 (95% CI: 1.07, 6.12) and 1.76 (95% CI: 1.04, 3.01) for those without and with a history of respiratory allergy, respectively. The association of asthma with waist circumference was not statistically significant in all the subgroups defined by atopy and respiratory allergy. The data suggested a stronger association between obesity and asthma among nonatopic people than among atopic people.