The public health impact of returning to school after vacations on children's asthma have not been well explored. In this study, we investigated the timing and magnitude of New York State asthma hospitalizations from 1991 to 2001. We used a generalized additive model to assess the risk associated with school return among children compared to adults and the elderly. Hospital admissions on the first day of school were compared to days following four school breaks. Exceedance admissions and both length and cost of hospital stay were estimated. A significant increase in hospital admissions for asthma (20% to 300%) was associated with school return after each break. The strongest associations were observed following summer vacation and for children age 5 to 11 years. Returning to school after vacations substantially increases the risk of hospital admissions for asthma in children, and this has considerable public health and economic impact.