The number of children with asthma is rising. This is shown by a study of medically diagnosed asthma in a cohort of children born in 1990 in a well defined and limited region. The medical records from all of the health centres in primary health care, privately practising paediatricians and relevant child clinics in the region were examined when the children were 7 years old. The results showed that 10.9% of the children had been diagnosed with asthma. The percentage of children with asthma was highest (14.2%) among boys living in a city environment. Comparison with a similar follow-up study in the same region concerning children born in 1975 showed that the percentage of children with asthma doubled during a period of just over a decade.