This study aimed to identify the rate of surgery cancellation and to verify the reasons for patients' absence from the planned surgery. The study was carried out at a secondary care university hospital in São Paulo city. Sixty patients whose surgeries were cancelled were interviewed during 3 months. Results showed that the rate of surgery cancellation was 19.91%, and that absenteeism was responsible for 54.30% of the 186 cancelled surgeries. Ten reasons for patient absenteeism were found; 53.33% were due to institutional problems, in most cases because patients were unaware of the date of surgery; 28.33% had clinical problems such as upper airway infection and other diseases; social conditions were responsible for 10.00%, related to economic and family problems; and 8.33% were cancelled by the patient.