The prevalence of occupational asthma has usually been estimated from registers of cases rather than population surveys. We examined the causes and derived estimates of the risk of asthma attributable to occupational exposures in a randomly selected population of five areas of Spain. The study is part of the EC Respiratory Health Survey and comprises 2,646 subjects age 20 to 44 yr. Bronchial reactivity was determined in 1,797 subjects and atopy in 2,164. Twenty-one occupational sets were defined using information on current occupation, or in subjects reporting change of occupation due to respiratory problems, their occupation at that time. The highest risk of asthma was observed for laboratory technicians, spray painters, bakers, plastics and rubber workers, welders, and cleaners. The risk of asthma attributed to occupational exposures after adjusting for age, sex, residence, and smoking status was 5.0% when asthma was defined as "bronchial reactivity and a report of wheezing or whistling in the chest during the last 12 mo," and 6.7% when asthma was defined as "bronchial reactivity and a report of asthma-related symptoms or medication." Estimates of the attributable risk for adult onset asthma were higher. Occupational exposures constitute a substantial cause of asthma in the young adult Spanish population.