The prevalence was assessed in a randomized group of 18-year-old and older inhabitants of Prague 8. A total of 8458 internationally tested questionnaires were sent out to a population of 90,458; information was obtained on 7605 (89.9%), 1274 were given a detailed examination by standardized methods. Cumulative prevalence of bronchial asthma (B): 2.3% +/- 0.3% (same in M and F). The highest values at the age of 18-20 years were 3.5%; 3.6% in the age group 21-30 years, the absolutely highest values--4.6%--were recorded in 18-year-old men. Contemporary prevalence of BA: 1.99%. In BA bronchial obstructions were found in 51%, bronchial hyperreactivity in 75%, positive skin tests in 71%, an increase of IgE in 36%. Prevalence of chronic bronchitis (CHB): 14.1%; men 17.6% women--11.3%, highest value--18.9%--in the age group 70-80 years; absolutely highest values were recorded in men aged 70-80 years--20.9%. In CHB there was a bronchial obstruction in 55% and this obstruction was reversible in 40%. In the group CHB bronchial asthma was recorded in 20%. Prevalence of bronchial hyperreactivity (BHR) was estimated from results of broncho-provocative tests and from the calculation according to prevalence of BHR in respiratory diseases. The thus calculated figure was 13.7%. The incidence of bronchial asthma in chronic bronchitis in 1/6 and of bronchial hyperreactivity in more than 1/10 of the adult population are striking facts. It is essential to find the causes of these findings and make effective provisions to counteract them.