Background: Clinical decision-making skills of paramedics have been emphasized because of the growing complexity of emergency medicine nursing. A preliminary diagnosis made by a paramedic has an essential role in directing the subsequent care. An accurate preliminary diagnosis improves the patient's outcome. The research in this area is relatively scarce and there are no previous studies in Finland describing the accuracy of preliminary diagnoses made by paramedics. The aim of this study was to evaluate whether paramedics are making accurate preliminary diagnoses for the patients they are transporting to hospital. In addition, the aim was to describe the variables related to an accurate preliminary diagnosis. Methods: A cross-sectional comparative approach was used and conducted through a questionnaire to gather data from the paramedics. A total of 71 paramedics participated in the study and 378 patient cases were included. The paramedics were asked to describe the basic information of a case, to state their preliminary diagnosis, and give their own educational background. The accuracy of the paramedic's preliminary diagnosis was compared with the discharge diagnosis of the ED physicians retrieved from hospital's patient records. Logistic regression analysis and a binomial test were used to test the statistical significance. Results: The agreement between the paramedics' preliminary diagnosis vs. hospital diagnosis was 70% (n = 261). Diagnostic accuracy varied according to the medical condition from mental diseases and intoxication (86%, p = 0, 000), cerebral strokes (81%, p = 0,007) to infections (31% p = 0,029). The educational background of a bachelordegree-level paramedic (p = 0,016, 95% Cl 1,7-139,6) and a good self-assessment value (p = 0,003, 95% Cl 1,2-2,7) were related to making a correct diagnosis. Conclusions: Paramedics are able to determine preliminary diagnoses at satisfactory level. The relationship between educational background and diagnostic accuracy suggests that there is a definitive need for a specific prehospital nursing education.