The purpose of this paper is to examine personal and health factors, both at the beginning of the study and thereafter, associated with participation in the GAZEL cohort, set up in 1989 in a large French company. The authors used logistic regression to analyze the associations between participation and data available for both participants (n = 20,093) and nonparticipants (n = 24,829). Higher participation was associated with male sex, marriage, children, managerial status, and residence in particular regions. Among men, lower participation was associated with sick leave in the year before recruitment and afterwards. During follow-up, participation was negatively associated with several groups of diseases, especially those associated with alcohol consumption. The risk of upper respiratory and digestive tract and lung cancer during follow-up was higher among nonparticipants. The same phenomenon occurred among women, but less markedly, for cancers of the breast and genital organs. During follow-up, mortality among men was higher among nonparticipants, especially for alcohol-related diseases. The association among women was less strong. Among men, but not among women, diseases caused by alcohol, smoking, or dangerous behavior were the primary reason for the health differences observed between participants and nonparticipants. Overall, the most important determinants of participation were cultural factors and lifestyle behaviors.