OBJECTIVES To describe the seasonal pattern of hospital admissions for childhood asthma in Sydney, Australia and investigate the relationship between these admissions and time of return to school. STUDY DESIGN Time-series analysis of daily hospital admissions for childhood asthma in Sydney from 1994 to 2000. METHODS We defined the time series of all asthma-related hospital admissions in Sydney between 1994 and 2000 for age groups 1-4 and 5-14 years. We analysed the time series for each age group using a generalized additive model with a log-link function, an offset term and quasi-likelihood estimation. Daily admissions were modelled using penalised regression splines adjusting for long term trends, school terms and holidays, weekday and influenza epidemics. RESULTS After adjusting for potential confounding, the risk of asthma admission increased to a peak between 2 and 4 weeks after the first day of school in each term and varied between 1.5 and 3 times the risk prior to return to school for both age groups. The largest increase in asthma risk occurring in term one after the long summer holiday. The increase in admission risk began soon after the first day of school of each term for school age children 5-14 years, but not in pre-school age children 1-4 years. CONCLUSIONS Returning to school after term holidays is strongly associated with increased risk of hospital admissions for asthma in children, especially following the long summer holiday. Preventive measures focused on return to school have the potential to substantially decrease admissions for asthma in children.