Prevalence of bronchial asthma and bronchial hyperreactivity (BH) was studied in a randomized sample of Prague 8 inhabitants aged 19 and over. The number of adults living in Prague 8 had been 90 458 by December 31, 1987. 8458 questionnaires had been sent to 5 randomly selected districts. Information mainly concerned with respiratory diseases was received from 89.9% persons. Cumulative prevalence (i.e. both contemporary and past) of bronchial asthma was 2.3% +/- 0.3% in both sexes. The cumulative prevalence calculated by way of direct standardization was slightly higher - 2.33% +/- 0.33% (men 2.33% +/- 0.5%, women 2.32% +/- 0.45%). The highest value of 3.6% was found at the age of 21-30 years. Evidence of BH was found in 75% cases of bronchial asthma by means of the inhalation histamine test. In adult population, the BH value was calculated on the basis of distribution of incidence in the healthy population and in persons with respiratory disease and the BH prevalence was found in each group. The BH in total adult population of Prague 8 was 13.7%. This is an alarming figure.