BACKGROUND: There is accumulating evidence that obesity is associated with an increased risk of asthma. It has been hypothesized that insulin resistance may be involved in obesity-induced asthma, but till date there is no prospective data on this issue. OBJECTIVE: To investigate the association of obesity and insulin resistance with the incidence of asthma-like symptoms in adults. METHODS: Out of a random sample of 12 934 persons from a general population, 6784 (52.5%) were included and participated in a health examination in 1999-2001. After 5 years they were re-invited and 4516 (66.6%) participated at follow-up. At baseline three obesity measures were considered: body mass index, waist circumference, and waist-to-hip ratio. In addition, fasting glucose and insulin were measured for determination of insulin resistance. Information on asthma-like symptoms at baseline and follow-up were obtained by questionnaires. A total of 3441 participants defined as non-asthmatic at baseline and with complete information on all the considered variables were included in the analyses. Data were controlled for confounding by sex, age, social status, and smoking. RESULTS: All obesity measures were associated with incident wheezing and asthma-like symptoms. In addition, insulin resistance was associated with incident wheezing [odds ratio (OR) 1.87, 95% confidence interval (CI) 1.38-2.54] and asthma-like symptoms (OR 1.61, 95% CI 1.23-2.10). The effect of insulin resistance was stronger than that of obesity and was independent of sex. CONCLUSION: We found that insulin resistance was associated with an increased risk of developing asthma-like symptoms. This finding supports the hypothesis that obesity and asthma may be linked through inflammatory pathways also involved in insulin resistance.