Criminal justice practitioners attribute the growth to arrests, which rose from 17,238 to 23,832 per month during the same 5-year period. But the authors warn that relying on arrest trends to explain the rise in detention population can be misleading. They argue that decisions about bail and release made at the arraignment hearing determine who goes to jail, and that these decisions affect the size of the pretrial detention population, regardless of changes in arrest volume. For example, the recent shift toward higher detention rates for drug offenders has affected the city's detention population. The implications of these findings for policymakers responsible for the projection of future jail population and the management of scarce detention resources are discussed. 23 footnotes.