Recent research suggests that pollution has a large impact on asthma and other respiratory and cardiovascular conditions. But this relationship and its implications are not well understood. I use changes in location due to military transfers, which occur entirely to satisfy the needs of the army, to identify the causal impact of pollution on children's respiratory hospitalizations. I use individual-level data of military families and their dependents, matched at the zip code level with pollution data, for the period 1989-95. I find that for military children only ozone appears to have an adverse effect on health, although not for infants. (Contains 5 figures, 7 tables, and 34 footnotes.)