BACKGROUND: Traffic-related pollution is associated with the onset of asthma in children. Its effect on adult-onset asthma is poorly investigated. The SAPALDIA cohort study was used to investigate associations between the 11-year change (1991-2002) in home outdoor traffic-related particulate matter up to 10 microm in diameter (TPM(10)) and the incidence of asthma. METHODS: Never-smokers without asthma at baseline aged 18-60 years in 1991 were eligible for inclusion in the study. Subjects reporting doctor-diagnosed asthma at follow-up were considered incident cases. TPM(10) at baseline and follow-up was predicted and interpolated to subjects' place of residence by dispersion models using emission and meteorological data. Cox proportional hazard models for time to asthma onset were adjusted (age, gender, baseline atopy, body mass index, bronchial reactivity, maternal allergies). RESULTS: Of 2725 never-smokers, 41 reported asthma onset in 2002. Home outdoor TPM(10) concentrations improved during the interval (mean -0.6; range -9 to +7.2; IQR 0.6 microg/m(3)). The incidence of asthma was associated with a change in TPM(10). The hazard ratio (1.30; 95% CI 1.05 to 1.61) per 1 microg/m(3) change in TPM(10) (IQR) was not sensitive to further adjustments (education, workplace exposure, passive smoking, parental asthma or allergies, random area effects, lung function or co-pollutants such as regional, secondary, total PM(10) or proximity to busy roads). CONCLUSION: The data suggest a role for traffic-related pollution in adult-onset asthma. Space, time and source-specific individual assignment of exposure to traffic-related pollution is a key strength of SAPALDIA. It may explain why findings were statistically significant despite the limited number of new cases. As traffic-related pollution prevails, the finding may be of substantial public health relevance.