The purpose of this study was to generate national estimates of the prevalence of medically unnecessary emergency medical services (EMS) transports to emergency departments (EDs) over time and to identify characteristics that may be associated with medically unnecessary transports. A previously published algorithm was applied to operationalize medical necessity based on ED diagnosis to 10 years of data from the National Hospital Ambulatory Medical Care Survey. The trend over time was reported using descriptive statistics weighted to produce national estimates. Nationally, the proportion of EMS transports that were medically unnecessary increased from 13% to 17% over the 10-year study period. Individual demographic characteristics, including insurance status, were not predictive of inappropriate utilization. EMS transports for medically unnecessary complaints increased from 1997 to 2007. Our findings from a nationally representative sample highlight the opportunity for alternative patient delivery strategies for select patients seeking EMS services.