Much research on utilization of hospital emergency departments has been published over the past 10 to 15 years. It has failed to yield a coherent view of why the volume of use has increased, however, because most of it has focused on users of one or more ERs, ignoring the nonusers, and has provided insufficient detail about the local context in which the ER operates. The result has been large quantities of data which, when compared, produce inconsistencies which cannot be resolved without additional data from different studies. Yet, a tentative explanation of ER growth can be presented if the question of why people use ERs, which is usually thought of as being similar to the question of why people use medical care services, is restated as, why do people who want to use medical care choose the ER as the site of care? That question can best be answered by paying greater attention to enabling and illness factors than to the predisposing demographic factors upon which much research has focused. A tentative explanation of the growth of ER utilization is offered. Then, the support from the literature for it is presented and the remaining questions are identified for future research.