The computer records of Swedish conscript examinations in 1971 and 1981 were analysed regarding the prevalence of asthma and allergic rhinitis. The material comprised ∼ 55 000 18-year-old males in each cohort. During the 10-year period the prevalence of asthma increased from 1.9 to 2.8% and of allergic rhinitis from 4.4 to 8.4%. The prevalence of allergic rhinitis was already higher in the northern than in the southern part of the country in 1971. The increase in both diseases was higher in the northern parts of Sweden, giving double prevalence figures in the northern part of the country compared to the southern part in 1981. The distribution of the prevalence was similar to the temperature pattern in the country, the higher prevalence being associated with a colder outdoor climate. Both diseases were more prevalent in urban than in rural areas.