- OBJECTIVES: Seasonal variations in asthma are widely recognized, with the highest incidence during September. This retrospective population study aimed to investigate whether this holds true in a large group of asthmatic children in primary care and to assess the impact of age, gender, urban/rural living, and population sector. METHODS: The key study outcomes were the diagnosis of asthma exacerbations and asthma medication prescriptions, recorded by family physicians during 2005 to 2009. These were analyzed by “week of diagnosis” in Clalit Health Services’ electronic medical record database. Regression models were built to assess relative strength of secular trends, seasonality, and age-group in explaining the incidence of asthma exacerbations. RESULTS: A total of 919 873 children aged 2 to 15 years were identified. Of these, 82 234 (8.9%) were asthmatic, 61.6% boys and 38.4% girls; 49.1% aged 2 to 5 years, 24.1% 6 to 9 years, and 26.8% 10 to 15 years. We observed a 2.01-fold increase in pediatric asthma exacerbations and 2.28-fold increase in prescriptions of asthma bronchodilator medications during September (weeks 37–39 vs weeks 34–36) compared with August. The association between the opening of school and the incidence of asthma-related visits to the primary care physician was greatest in children aged 2 to 5 years (odds ratio, 2.15) and 6 to 11 years (1.90-fold). Adolescents (age 12–15 years) had a lesser peak (1.81-fold). In late fall there was a second rise, lasting with fluctuations throughout winter, with a trough in summer. CONCLUSIONS: Returning to school after summer is strongly associated with an increased risk for asthma exacerbations and unscheduled visits to the primary care physician.