The prevalence of chronic bronchitis and bronchial asthma has been studied in a community in northern Sweden, where the major air pollutant is sulphur dioxide (SO2) from a sulphite pulp factory. In the town the annual mean winter concentration of SO2 during the years 1975-1979 was 20-40 micrograms SO2/m3 air. A questionnaire concerning respiratory symptoms was distributed to all individuals (2374) within the age group 16-72 years, and was answered by 2203 individuals (93%). Bronchial asthma was found in 78 (3.5%) individuals, chronic bronchitis in 57 (2.6%) and both diagnoses in 3 individuals (0.1%). There was a statistically significant relationship between presence of chronic bronchitis and sex, age, smoking and employment at the sulphite pulp factory. Sulphur dioxide exposure at the factory apparently acted synergistically with smoking in increasing the relative risk for chronic bronchitis in smoking employees compared to non-smoking non-employees. However, our results indicate no excess risk of chronic bronchitis in a population with long-term environmental exposure to SO2 in concentrations between 20-40 micrograms, when the concentration of other air pollutants is low.