This population-based cross-sectional survey assessed the prevalence of work-aggravated asthma symptoms and the effect of the work environment on the aggravation of symptoms of established asthma. A questionnaire was sent to 2,613 persons (aged 20-65 yrs) with asthma. The analyses were restricted to the 969 respondents who were currently employed. The effect of occupational exposure on the aggravation of asthma symptoms at work was assessed according to both self-reported and expert-evaluated exposure. Approximately 21% of the respondents reported work-aggravated asthma symptoms at least weekly during the past month. The prevalence of those with work-aggravated symptoms increased by age, self-reported occupational exposure to dusts, abnormal temperatures or poor indoor air quality, physically strenuous work, and chemicals, and expert-evaluated probability of daily occupational exposure to airborne dusts, gases or fumes. Aggravation of asthma symptoms at work is common among employed adults with asthma. Both self-reported and expert-evaluated exposure to dusts, abnormal temperatures or poor indoor air quality, physically strenuous work, and chemicals explained the significant worsening of symptoms. The findings suggest a marked role of the work environment in the aggravation of symptoms of established asthma.