Introduction: Cancellations of elective operations on the day of the surgery reflect the efficiency and quality within organisations, and have a significant clinical, social and economic impact, not only for the patient and their families, but also for healthcare institutions. This study assesses the extent of these cancellations in one public Portuguese hospital, through case quantification and identification of the causes, origin, as well as its predictability according to the sociodemographic variables of the patient and interventions used to decrease it.Material and Methods: Non-experimental descriptive quantitative methodology – longitudinal and retrospective – of operation cancellation cases on the day of the surgery, from the 1st of January to the 31st of December 2018.Results: The rate of cancellations of elective surgeries on the same day of the operation was 2.9% with variations among different surgical specialties; cancelled operations are more frequent in female patients, in patients aged between 50 and 80 years old, physical status classified as II or III according to the American Society of Anesthesiology, and without anaesthetic pre assessment or preoperative consultations; the three most relevant causes for cancellations are: lack of operative time, scarcity of beds and/or medical equipment, and changes in health status; most of which can be avoided and are the responsibility of the institution.Discussion: Different reasons for cancellation of elective operations reflect a variety of upstream and downstream processes causing cancellation of surgeries and whose origin/imputability is related to both the institution and patients.Conclusion: The rate of cancellations of elective surgeries on the same day of the operation is relatively low, but the causes are often preventable, thus justifying the generalization of cancellation reduction strategies.