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OBJECTIVES--To estimate changes in the prevalence of reported symptoms of respiratory disease and reported diagnoses of asthma and bronchitis in primary school children in England between 1973 and 1986. DESIGN--Mixed longitudinal survey. SETTING--Representative sample of English primary schools in 22 areas. PARTICIPANTS--15,000 Boys and 14,156 girls each studied at least once between 1973 and 1986. DATA COLLECTED--Whether, according to the parent or guardian, the child had wheezed, wheezed on most days or nights, or had attacks of bronchitis or asthma in the past year. RESULTS--Within age groups trends in successive annual cohorts showed an increasing prevalence of asthma for each annual birth cohort (boys, 6.9%, p less than 0.001; girls, 12.8%, p less than 0.001) and of wheeze on most days or nights (boys, 4.3% per cohort, p less than 0.001; girls, 6.1% per cohort, p less than 0.001) and a falling prevalence of bronchitis (boys, -4.7% per cohort, p less than 0.001; girls, -5.8% per cohort, p less than 0.001). There was a smaller increase in the prevalence of wheeze whether or not it occurred on most days or nights, and this increase was significant only among the girls (boys, 1.0% per cohort, p greater than 0.05; girls, 1.7% per cohort, p less than 0.05). Although the rate of increase of "asthma" was greater than the rate of decrease in "bronchitis," the baseline prevalence of asthma was much lower than that of bronchitis, and the total proportion of children with either diagnosis declined slightly over the whole period. The main change was an increase in the proportion of children whose parents stated that they had persistent wheeze and yet did not have a report of either "asthma" or "bronchitis." CONCLUSIONS--These results suggest that there has been a true increase in morbidity that is not simply due to changes in diagnostic fashion. The increase is large enough to explain much if not all of the increase in admission to hospital and mortality, and it underlines the importance of an understanding of the aetiology of asthma in tackling the causes of the recent increase.