The prevalence of asthma and allergic rhinitis was analysed in relation to birth month in two cohorts of 18-year-old males born in 1953 (n= 55 393) and 1963 (n= 57 150), in school children (n= 1984) and in children with defined specific allergies (n= 1080). Among the 18-year-olds, a marked variation with respect to birth season was only found in those born in 1963, with the highest prevalence of asthma in those born August-January and of allergic rhinitis in those born November-May. In the schoolchildren with allergic rhinitis the birth season pattern was found to be similar to that in the conscripts born in 1963. The influence of month of birth on the onset of allergic rhinitis was maximum about 6 years of age and was not seen before 2, and after 10 years of age. Obvious similarities in birth season patterns were found between the unselected populations and the children with defined pollen and animal dander allergies. The differences in birth season pattern between the two cohorts of 18-year-olds were interpreted as an effect of unidentified adjuvant factors that might have developed over a 10-year period.