Rationale, aims and objectives: The objective of this study was to develop a questionnaire measuring the level of trust and its constituents in patients calling the Emergency Medical Services (EMS) for suspected acute primary healthcare problems. The questionnaire is called the Patient Trust Questionnaire (PTQ). The following frontline service providers were involved: (1) The Dispatch Centre, (2) the Emergency Medical Services, and (3) the receiving unit (Emergency Department/Healthcare Centre). Method: Cross-sectional data were collected repeatedly and redundant items were discarded using a step-by-step approach. Based on a literature review, the PTQ was developed in line with the following 4-step procedure: (1) Item construction, (2) a face-to-face evaluation of separate items, (3) an empirical pre-evaluation targeting each separate frontline service provider and (4) an empirical full-scale evaluation. The inclusion criteria for participating were that the patient must be 18 years of age or older and suspected having an acute primary healthcare problem when calling the EMS. In the final full-scale evaluation of the questionnaire, 427 patients were included. Results: A set of 8 items with good psychometric properties remained through the developing procedure. Two constituents of trust emerged (labelled credibility and accessibility), which were robust across all frontline service providers. Conclusion: A new measuring instrument has been developed for this particular healthcare chain, for patients with suspected acute primary healthcare problems calling the EMS.Although not yet validated, the PTQ is a potentially useful tool in future healthcare research with reference to the concept of patient trust.