As for other chronic diseases, surveys of allergic conditions can give useful data about their prevalence and natural history. A study of the frequency of major allergic manifestations--asthma, allergic rhinitis, hay fever, atopic dermatitis and urticaria--could be performed twice at 13 years interval (1968/1981) among kindergarten children (4-6 years old) and 9th grade students (15 years old) attending the Geneva public schools. These samples represent about 90% of the corresponding age groups of the total population. Both surveys were conducted with individual interviews and physical examinations by trained physicians and nurses. The total prevalence of allergy was 5.4% in 1968 and 7.0% in 1981 for the children, and 10.3% and 11.5% for the adolescents. Asthma prevalence was 1.7% in 1968 vs 2.0% in 1981 among children, and 1.9% vs 2.8% among adolescents. For the other diseases the figures are: allergic rhinitis 0.6% vs 0.2% and 1.0% vs 0.6%; hay fever 0.5% vs 1.1% and 4.4% vs 6.1%; atopic dermatitis 2.2% vs 2.8% and 2.3% vs 1.5%; urticaria 0.4% vs 0.9% and 0.7% vs 0.5%. The rate increases over the years concern mainly the documented respiratory manifestations of atopy. Variables like sex, family atopy, ethnic origin and socio-economic status seem to be important factors influencing prevalence. Environmental factors may explain the increase of allergy in childhood and adolescence.