Registration of complications in surgery is an important method used for quality improvement. Unfortunately many different definitions and classification systems have been used, which influences the interpretation and the outcome of complication registration. Since 1986 complications have been registered on a daily basis in our surgical department. We focus in this article on the influence of changes in interpretation of the definition and registration methods used on the incidence of registered complications. Between 1986 and 1993 complications registered were strictly related to surgical procedures. In the second period, between 1993 and 2001, the interpretation of the definition changed and all adverse events were registered in a patient-centred way, not only related to the surgical procedure. The definition used in both periods did not change. In 1993 we started with the implementation of a fully automated registration system in our surgical department. In the first period 1699 (7%) complications in 24,201 surgical procedures were registered and in the second period 8335 (27%) complications were registered in 31,161 surgical procedures. A dramatic increase in the total number of registered complications was seen with the implementation of a fully automated registration system and a patient-centred way of registering complications. In the context of the evolving discussion of quality of care, a uniform definition and registration system has to be used to assure reliable outcome data in surgery and to form a basis for comparison.