We examined the relation between serum-free testosterone and asthma, wheeze, asthma hospitalisations and lung function in older adults.Cross-sectional study.UK.256 419 adults aged 40 to 69 years, recruited from 2006 to 2010.Multivariable logistic or linear regression was used for the analysis of free testosterone and physician-diagnosed asthma, current wheeze, asthma hospitalisations and lung function measures, which was adjusted for serum estradiol, smoking status and other covariates.Free testosterone levels above the lowest quartile (Q1) were significantly associated with lower odds of asthma in both women (adjusted OR (aOR) for Q4 (the highest quartile) versus Q1=0.67, 95% CI=0.64 to 0.71) and men (aOR for Q4 versus Q1=0.87, 95% CI=0.82 to 0.91). Among subjects with asthma, free testosterone levels above Q1 were significantly associated with lower odds of current wheeze in women (aOR range=0.78 to 0.87), and free testosterone levels in Q4 were associated with lower odds of current wheeze in men (aOR for Q4 versus Q1=0.86, 95% CI=0.77 to 0.95). Among women with asthma, free testosterone levels in Q4 were also associated with lower odds of ≥1 asthma hospitalisation. Among men, free testosterone was positively associated with FEV1 and FVC. Among women, free testosterone was negatively and weakly associated with FVC.In a large study of British adults, elevated free testosterone levels are associated with lower odds of asthma and current wheeze in women and men, lower odds of asthma hospitalisations in women, and higher FEV1 and FVC in men. DISSEMINATION TO PARTICIPANTS, AND RELATED PATIENT AND PUBLIC COMMUNITIES: The results of the study will be linked to the UK Biobank website.