The objective of the present study was to evaluate the workload and case-mix of the Emergency Department of a referral hospital, with the aim of determining the causes of overcrowding. The study was of a descriptive prospective design and was carried out in a 700-bed university hospital covering a population of approximately 200,000 inhabitants. The total number of patient visits to the Emergency Department and hospital admissions were recorded during one year, whereas the case-mix of patients visiting the department was evaluated during 11 consecutive on-call days using a triage scale with four categories of patient severity. During the year of the study 81,110 patients (a mean of 443 visits per on-call day) visited the department, with a hospital admission rate of 14.5%. The case-mix analysis of 3389 patients who visited the department during the 11 consecutive on-call days studied revealed that 5.7% were critically ill patients, 53.8% were patients with urgent but non-critical health problems, 30% were patients with non-urgent problems and 10.5% were miscellaneous cases, probably inappropriately visiting the department. In conclusion, the Emergency Department studied is severely overcrowded in relation to the population that it covers. A great part of this overload was directly related to non-urgent cases and inappropriate visits.