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Evidence on variability in emergency medical services use is limited. We obtain national evidence on geographic variation in the use of ambulance transport to the emergency department (ED) among Medicare enrollees and assess the role of health status, socioeconomic status, and provider availability.We used 2010 Medicare claims data for a random sample of 999,999 enrollees aged 66 years and older, and identified ambulance transport and ED use. The main outcome measures were number of ambulance transports to the ED per 100 person-years (ambulance transport rate) and proportion (percentage) of ED visits by ambulance transport by hospital referral regions.The national ambulance transport rate was 22.2 and the overall proportion of ED visits by ambulance was 36.7%. Relative to hospital referral regions in the lowest rate quartile, those in the highest quartile had a 75% higher ambulance transport rate (incidence rate ratio [IRR] 1.75; 95% confidence interval [CI] 1.69 to 1.81) and a 15.5% higher proportion of ED visits by ambulance (IRR 1.155; 95% CI 1.146 to 1.164). Adjusting for health status, socioeconomic status, and provider availability reduced quartile 1 versus quartile 4 difference in ambulance transport rate by 43% (IRR 1.43; 95% CI 1.38 to 1.48) and proportion of ED visits by ambulance by 7% (IRR 1.145; 95% CI 1.135 to 1.155). Among the 3 covariate domains, health status was associated with the largest variability in ambulance transport rate (30.1%), followed by socioeconomic status (12.8%) and provider availability (2.9%).Geographic variability in ambulance use is large and associated with variation in patient health status and socioeconomic status.