Diet is considered an important aspect of lifestyle related to cancer development. To contribute further knowledge within this field a Danish prospective cohort study ``Diet, Cancer and Health'' has been initiated. The aims of this paper are to give a description of the study design, measurement procedures, and differences between participants and nonparticipants with special reference to socioeconomic characteristics. A total of 160,725 individuals 50—64 years of age living in Copenhagen or Aarhus were invited to participate. Information concerning diet and other lifestyle factors was obtained from 57,053 participants using questionnaires and interviews. Anthropometric measurements were taken and biological material collected. In addition, detailed (selected) socioeconomic information on all invited persons including 103,671 non-participants was obtained from statistical registers in Statistics Denmark. Differences were seen between participants and non-participants on a number of socioeconomic factors. The highest participation in relation to education was found among participants with higher education, with a significant tendency to be highest in the second highest level of higher education (3—4 years). Married people were more likely to participate than persons living alone or cohabiting. Results from the prospective cohort study ``Diet, Cancer and Health'' support the general assumption that lower socioeconomic groups are underrepresented in epidemiological studies.