The Danish Cancer Registry is a population-based registry containing data on the incidence of cancer throughout Denmark since 1943. Reporting of cancer was made mandatory by administrative order in 1987. Details of individual cases of cancer are available according to the 7th revision of the International Classification of Diseases (ICD) for all years, and according to the ICD-O since 1978. A core data set is kept on each individual which includes date of birth, sex, date of cancer diagnosis, method of verification, date of death and cause of death. This paper describes the history of the registry, its data sources and its procedures, including quality control and access to data. Integration of both research activities and registration since the inception of the Registry has maintained the completeness and validity of the data for 1943-1996.