This thesis focuses on the contemporary way in which telephone triage is carried out with the ‘Netherlands Triage Standard’ in callers with acute cardiovascular symptoms suggestive of transient ischaemic attack (TIA)/stroke or acute coronary syndrome (ACS) who call the out-of-hours service in primary care. (i) The ‘Netherlands Triage Standard’ performed moderately regarding safety (sensitivity) and poorly regarding efficiency (specificity) in allocating adequate urgencies to patients with and without TIA/stroke. (ii) We developed a new diagnostic prediction model for detecting TIA/stroke with telephone triage at the out-of-hours service in primary care, which had good discriminative ability with as strongest predictors age, female sex, arm weakness, and speech problems. (iii) Missed ACS patients largely overlap in characteristics with patients with symptoms suggestive of ACS but without such a serious adverse event. We could not identify characteristics of missed ACS that could be helpful for triage to help prevent future adverse outcomes in patients with chest discomfort. (iv) General practitioner experts rated - blinded to the outcome - the telephone triage of missed ACS patients more often as ‘unsafe’, and of ‘poor’ quality than that of patients with symptoms suggestive of ACS but without a serious adverse event. However, the level of agreement among experts was low. (v) Conversation analysis showed that the use of multiple-choice either/or-questions (e.g. “Is it an oppressive or stabbing or stinging pain?”) by the triage nurse can lead to difficulties in the interaction between the triage nurse and the caller.