Socioeconomic position and lifestyle often affect participation in scientific studies. The authors investigated differences in overall and cause-specific mortality between participants and non-participants in the prospective Danish cohort study "Diet, Cancer and Health" and the association between non-participation and mortality by socioeconomic position. A total of 80,996 men and 79,729 women aged 50-64 years, were invited. The authors obtained register data on education, income, death and cause-specific mortality for participants and non-participants and used survival curves to examine differences in overall mortality. Poisson regression models were used to estimate the mortality rate ratio (MRR) by socioeconomic group and by cause of death of participants and non-participants. After a median follow-up of 13 years (5-95 percentiles, 5-14 years), the MRRs for overall mortality among non-participants were 2.09 (95% CI 1.99-2.14) and 2.29 (95% CI 2.19-2.40) among men and women, respectively compared with participants. After adjusting for socioeconomic position, the MRRs changed to 1.73 (95% CI 1.66-1.79) and 2.10 (95% CO 2.01-2.20) among men and women, respectively. The MRRs did not level out after up to 15 years of follow-up. The MRRs were all significantly increased and ranged from 1.51 to 4.28 for men, depending on the cause of death, and from 1.60 to 3.99 for women. Clear differences in mortality from all investigated causes of death were found between participants and non-participants, which persisted after up to 15 years of follow-up. Socioeconomic position had little effect on this result.