Study objectives: Emergency medical services (EMS) is an important part of the health care system. The effect of EMS on morbidity, mortality, and costs of illness is difficult to evaluate because hospital information is not available in out-of-hospital databases. We used probabilistic linkage to create such a database from ambulance and inpatient data and demonstrate the potential for linkage to facilitate evaluation of EMS responses resulting in hospital admission. Methods: Statewide ambulance and inpatient hospital discharge records were available for 1994 through 1996. Ambulance records indicating admission to the emergency department or hospital (165,649 records) were linked to inpatient hospital records indicating emergency admission (146,292 records) by using probabilistic linkage. Out-of-hospital data (dispatch code, treatments rendered, and ages), linkage rates, and inpatient data (discharge status, charges, length of stay, and payer category) were analyzed. Results: We linked 24,299 (14.7%) ambulance events to inpatient hospital discharges. If we had used exact linkage methods, we would have only linked 14,621 record pairs, a loss of nearly 40%. Linkage rates were relatively constant between years (approximately 15%) but differed by ambulance dispatch codes. Out-of-hospital dispatch codes with high linkage rates included breathing problems (22.6%), chest pain (21.5%), diabetic problems (16.9%), drowning incidents (14.9%), falls (19.2%), strokes (32.8%), and unconsciousness or fainting episodes (16.1%). Linkage to the hospital record provided access to hospital outcome data. Inpatient mortality was 6.8%. Survivors were discharged home (60.7%), transferred to other acute-care facilities (3.6%) or intermediate-care facilities (23.3%), or discharged with home health care provision (4.9%). The median length of stay was 3 days, and median charges were $6,620; total inpatient charges were $286,737,067. Conclusion: Probabilistic linkage enables ambulance and hospital discharge records to be linked together and potentially increases our ability to critically evaluate EMS by providing access to hospital-based outcomes. Such evaluation will be further improved by linking to ED, other outpatient, and other public health data sources. [Dean JM, Vernon DD, Cook L, Nechodom P, Reading J, Suruda A. Probabilistic linkage of computerized ambulance and inpatient hospital discharge records: a potential tool for evaluation of emergency medical services. Ann Emerg Med. June 2001;37:616-626.]