The task of evaluating incoming calls to Emergency Medical Services (EMS) systems in order to determine the most appropriate response is performed in many different ways in current EMS systems. At one end of the spectrum, the process is entirely dependent on the judgement of professionals, while at the other end protocols specify the exact questions to be asked and corresponding decisions. This case study describes the experience of the Montreal EMS system, Urgences santé, where professional telephone evaluation performed by nurses since 1981 was replaced by a protocolized system in 1992. During the professional era, there were many attempts to formalize the nurses' decision-making process. These first revealed that professional judgement tended to override decision-support tools that did not allow a flexible processing of the information spontaneously provided by callers. Second, the choice of a single protocol for each call was unnatural for professionals who could spontaneously integrate multiple aspects of a problem in parallel. Third, when protocols were used by professionals, it was a posteriori in order to document their decisions rather than actually support them. Fourth, the use of Artificial Intelligence (AI) methods in order to formalize professional judgement revealed its great complexity, which was confirmed by cognitive analyses of the nurses' decision-making processes. In particular, decisions of not sending EMS resources seemed to be the most difficult. These unsuccessful attempts at formalizing professional judgement led to an evaluation of its performance in terms of results, i.e. to which extent actual decisions minimized errors (both false positives and false negatives) and decision times. A random sample of 1006 calls was collected and the ideal decision was determined by concensus of experts for each call based on the patient's clinical condition. This theoretical decision was considered as a goal standard to which actual decisions were compared. Data analysis revealed that sensitivity of telephone triage (i.e. decision to send EMS resources or not) was almost perfect and specificity was 0.55. The necessary compromise between sensitivity and specificity varied with the types of decisions. Decision times were related to the urgency of the situations, more urgent calls being processed more rapidly. These results were interpreted as representing sophisticated optimization processes in professional judgement. The professional system was replaced by a non-professional protocolized system in 1992. This new system has not yet been formally evaluated in terms of results, but many sources of evidence suggest that it was accompanied by a deterioration of performance. Many contextual factors influence the organization of telephone assessment in EMS systems. This case study suggests that professional judgement may be most useful in contexts where the demand for EMS services often exceeds the availability of resources. On the other hand, protocolized systems may be more appropriate in the absence of such constraints, and where the litigation context prohibits the occurrence of any false negative.