This study examines the effects of a congestion tax in central Stockholm on ambient air pollution and the health of local children. We demonstrate that the tax reduced ambient air pollution by 5–15 percent and the rate of acute asthma attacks among young children. We do not see corresponding changes in accidents or hospitalizations for nonrespiratory conditions. As the change in health was more gradual than the change in pollution, it may take time for the full health effects of changes in pollution to materialize if the mechanism is pollution. Hence, short-run estimates of pollution reduction programs may understate long-run health benefits.