OBJECTIVES: Systematic reviews of the literature assist in the location, appraisal, and synthesis of available evidence. This systematic review aimed to 1) assess the existing literature evaluating the effect of the priority dispatch of emergency ambulances on clinical outcome and ambulance utilization and 2) assess the relative effectiveness of sources of literature relevant to prehospital care. METHODS: Systematic review. The quality of each paper was assessed using a standardized seven-point scoring schedule. Sources used were: Medline, the Cumulative Index to Nursing & Allied Health Literature (CINAHL), Bath Information & Data Services (BIDS), bibliographic searching, contacting researchers active in the field, and hand-searching relevant journals. Key words used were: "ambulance," "prioritisation," "dispatch," and "triage." RESULTS: Three hundred twenty-six papers were identified: 64 (19.6%) were related to the prioritization of emergency ambulances, and only 20 (6.1%) contained original data. The overall quality of publications was poor, seven (35%) papers having a quality score > or = 4. Only half were identified by electronic databases, 55% were identified by people working in the field, and two (10%) were identified by hand-searching (some papers were identified by more than one source). Two high-quality papers support the concept that criteria-based dispatch (CBD) improves clinical outcome; two other papers support CBD's role in improving ambulance utilization. CONCLUSIONS: There is very little evidence to support the effect of the prioritization of emergency ambulances on patient outcome. Electronic databases identify only approximately half of all relevant prehospital literature. Future systematic reviews in this area should use electronic databases, supplemented by contact with appropriate experts.