To assess the incidence of adverse events and patients with adverse events in ambulatory surgical procedures and to compare it with that of other studies.Historical cohort study. The scope of the study was the ambulatory surgical procedures unit of a university hospital. All general surgery department patients seen in this unit during the year 2005 were included.The incidence of patients with adverse events directly related to hospital care was of 3% (95% CI, 0.9-5). Of the adverse events identified 5 were considered slight, 3 moderate and none were considered serious. All the moderate ones were considered unavoidable and of slight, only the one was avoidable. Six of the adverse events were associated to a procedure, one due to hospital infections and one with other causes. There was an increase in hospital stay due to 75% of the adverse events, and 25% of them affected admission.The incidence of adverse events related to medical care in the Spanish hospitals is similar to those found in the studies carried out in American and European countries using the same methodology. The surgical area is considered a high risk unit. However, ambulatory surgical procedures reduce these risks, in such a way that the incidence is far below that of surgery department. Therefore, besides improving the technical efficiency of the clinical services, it is safer for the patients.