Although it is well known that West Coast residents spend 40% fewer days per year in the hospital than do East Coast residents, the reasons for this difference are less well understood. In this analysis of data from the 1977 National Health Interview Survey and Area Resource File, we attempted to isolate which share of the East-West difference is due to population factors and which share is due to characteristics of the health care systems. When we compared New York City hospital use with Los Angeles, population factors were slightly more important than health system characteristics. When we compared other Northeast metropolitan areas with Western metropolitan areas, health system factors were more important. In the case of the New York-Los Angeles comparison, we were able to isolate the following health system characteristics, in order of importance, as especially significant: the backlog of patients in acute care hospitals awaiting placement in long-term care facilities, the greater number of hospital beds per capita in New York City than in Los Angeles, and the larger supply of medical specialists and residents in New York City than in Los Angeles.