Background. Triage in the emergency department (ED) is necessary to prioritise management according to the severity of a patient's condition. The South African Triage Scale (SATS) is a hospital-based triage tool that has been adopted by numerous EDs countrywide. Many factors can influence the outcome of a patient's triage result, and evaluation of performance is therefore pivotal. Objectives. To determine how often patients were allocated to the correct triage category and the extent to which they were incorrectly promoted or demoted, and to determine the main reasons for errors in a nurse-led triage system. Methods. Triage forms from a tertiary hospital ED in Gauteng Province, South Africa, were collected over a 1-week period and reviewed retrospectively. Results. A total of 1 091 triage forms were reviewed. Triage category allocations were correct 68.3% of the time. Of the incorrect category assignments, 44.4% of patients were promoted and 55.6% demoted. Patients in the green category were most commonly promoted (29.4%) and patients who should have been in orange were most commonly demoted (35.0%). Trauma patients were more likely to be incorrectly promoted and non-trauma patients to be incorrectly demoted. Mistakes were mainly due to discriminator errors (57.8%), followed by numerical miscalculations (21.5%). The leading omitted discriminators were 'abdominal pain' , 'chest pain' and 'shortness of breath' . Conclusions. Mis-triaging using the SATS can be attributed to incorrect or lack of discriminator use, numerical miscalculations and other human errors. Quality control and quality assurance measures must target training in these areas to minimise mis-triage in the ED.