In this study, which is a part of a clinical-epidemiological study of post-operative complications, the Project Peri-operative Risk (PROPER), patients' and health professionals' perceptions of complications during the patients' stay in hospital are analysed. Five hundred and ten patients admitted for elective (= planned) surgery and attending a pre-operative outpatient clinic set up for this project were seen pre-operatively and then monitored closely during and after surgery. Three months after having been operated on, they were sent a postal questionnaire in which they were asked to indicate whether they had suffered post-operative complications and to evaluate the care process. The incidence of complications reported by patients and staff was similar, but there were large differences at the individual level. Two-thirds of the staff-reported complications were not reported by the patients and vice versa. Patients generally classified complications as more severe than the staff did. Patients' evaluation of the care process was clearly related to reported complications. Most dissatisfied were those patients who reported complications not reported by the staff, and the most satisfied were those who had complications according to the staff but no complications according to themselves. The implication of these findings is discussed.