In order to test hypotheses on diet and the risk of cancer, a prospective cohort study was established. A total of 57,055 persons living in Copenhagen and Aarhus, between 50 and 65 years of age, visited a study clinic between December 1993 and May 1997. The participants provided questionnaire data on diet and lifestyle. Furthermore, anthropometric measurements, blood pressure and biological material were collected. All participants will be followed by linkage to health registries including the Cancer Registry and by self-administered follow-up questionnaires. The purpose of this publication is to describe the data-base, which will be available for research in the years to come including the results of the first two years of follow-up.