The changing economy and greater public awareness of medical costs has created a need to assess efficacy, quality of care, and cost of prehospital treatment by paramedics. The purpose of the present study was to design a simple method for analysis of EMS function using a small number of variables to clearly identify parts of the system needing improvement. The method was used to evaluate EMS in a moderate-sized city over a 1-year period.The log books of paramedic activities and ambulance time records were reviewed and a microcomputer used to abstract the following information felt to comprise the minimum number of variables necessary to assess the system: 1) origin of call; 2) time of call; 3) case type; 4) response time; 5) time spent on scene; 6) transit time to hospital; 7) use of advanced life support (ALS) skills; 8) evaluation of overall run (Category A = Paramedics clearly needed; Category B = Paramedics probably of benefit; Category C = Paramedics clearly not needed, EMT perhaps of benefit; Category D = EMT not needed, first-aid treatment only required).In 1981, the Mobile paramedic system was accessed 4, 698 times. The false alarm rate was 80.4%. Of the 919 cases requiring treatment, 27%, 46%, 11%, 13%, 2%, and 1% were for major medical, minor medical, major trauma, minor trauma, major surgical, and minor surgical problems, respectively. Analysis of patients requiring treatment revealed that 25% of cases fell in Category A, 30% in Category B, 36% in Category C, and 8% in Category D. ALS skills were used in 82% of the 919 cases treated. This included two thirds of the Category C and D cases.Thirty per cent of the Category A cases occurred within 5 minutes of a fullystaffed emergency department; however, in 66% of these cases more than 20 minutes and in 15% more than 30 minutes were spent at the scene. Twenty-six per cent of the Category B cases were within 5 minutes of hospital, yet 32.3% of these cases required 20 minutes and 70% required more than 30 minutes in the field.Evaluation of the Mobile EMS System using this simple format has helped formulate the following priorities to improve cost benefit ratio, to: 1) decrease the false alarm rate; 2) improve the efficacy of EMS dispatchers; 3) improve the appropriateness of field use of ALS skills; 4) reevaluate paramedic training curricula.