BACKGROUND: Childhood is the most important age for asthma development. Recent reports indicate that the prevalence of asthma in children has plateaued after having increased for decades. AIMS: To study prevalence and risk factor patterns of asthma by age and by time. METHODS: In 1996, all children in grade 1-2 (age 7-8) in three cities in Northern Sweden were invited to an expanded International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire. A total of 3430 children (97%) participated yearly until 2000 (age 11-12). A subset (n = 2454) was invited to skin-prick testing in 1996 and 2000 with 88% and 90% participation. In 2006, another cohort (n = 2704) was identified and studied by identical methods with 96% participation. A total of 1700 children (90% of invited) were skin-prick tested. RESULTS AND COMMENTS: From age 7-8 to 11-12, the prevalence of physician-diagnosed asthma increased, 5.7%-7.7% (P