Hospital admission rates for childhood asthma have increased markedly in many countries since the beginning of the 1970s. Prevalence of asthma, as reported by parents or children in occasional surveys, has also increased during this period. In Montreal, 7.2 per 1000 3-year-old children were admitted to the hospital with a diagnosis of asthma in 1980-1981, whereas the rate was 11.9 per 1000 in 1984-1985. For 7-year-old children, these rates were 3.2 per 1000 and 4.8 per 1000, respectively. On the other hand, the length of hospital stay decreased steadily during this period, and the mean number of admissions per child did not show a tendency to increase. For 3- and 7-year-old children together, the prevalence of asthma, determined from health insurance data, increased by 71%, from 3.76% in 1980 to 6.45% in 1983. Prevalence of other medical conditions also increased in this community, but much less than for asthma. An increase in the incidence of the disease cannot be excluded as the cause for the changing prevalence of asthma.