The objective of this study was to evaluate the validity in Belgium of the National Triage Scale for judgement of the urgency of a patient's condition and making a case-mix description of the patient profiles in the different urgency categories. The study is of a descriptive retrospective and descriptive correlational design and was carried out in the emergency department at the University Hospital Gasthuisberg in Leuven, Belgium. The urgency of patients arriving at the emergency department was evaluated during one randomly selected shift a day over 12 weeks in 1997 by one of the four triage-educated nurses, using an instrument based on the National Triage Scale. Patient identification and outcome parameters were retrieved from the existing computer system. The data were mainly analysed using the Ridit analysis. Overall 3650 patients were evaluated: Category 1, 4.19%; Category 2, 24.44%; Category 3, 39.32%; Category 4, 27.97%; Category 5, 4.08%. Any similarity between sentinel diagnoses as well as between the admission percentages in this pilot study and the reference from Australia (Z = 0.827; p > 0.05) was noted. Different aspects influenced the triage nurses while determining the degree of urgency. Urgency categories profiles revealed a significant effect of age (Kruskall-Wallis = 530.5; p = 0.000). Higher categories of urgency resulted in a higher degree of admission (t (df = 3640) = 643.45; p = 0.000). It is concluded that a resemblance between the pilot study and the reference confirms the predictive validity of the scale used. Patient profiles in the different urgency categories give a description of the emergency department population.