This study examines dropout incidence, moment of dropout, and switching behavior in organized exercise programs for seniors in the Netherlands, as determined in a prospective cohort study (with baseline measurements at the start of the exercise program and follow-up after 6 months; N = 1,725, response rate 73%). Participants were community-living individuals 50+ who participated in different forms of organized exercise programs. The average dropout incidence was 0.15 per 6 months, which is lower than that for the general population. The dropout incidence and the timing of dropout differed substantially between the exercise programs. In total, 31% of people who dropped out of one type of exercise program switched to another type of exercise. The type of program and exercise had a strong effect on differences in this switching behavior. It is recommended that switching behavior be monitored in future studies.Key Words: physical activity, health behavior, health promotion, agingThere is clear evidence of the benefi ts of exercise programs for older adults (American College of Sports Medicine, 1998). Regular physical activity has been associated with a variety of health benefi ts such as improved cardiovascular, respiratory, and muscular function and reduction of risk factors associated with chronic disease states. Exercise also plays an important role in enhancing the quality of life of older adults by improving physiological and psychological function, which helps to maintain personal independence and reduces the demand for acute-and chronic-care services (Pate, et al., 1995; U.S. Department of Health and Human Services, 1996). It is therefore important to promote exercise participation among older individuals.Dropout from exercise programs is a problem (Carmody, Senner, Malinow, & Matarazzo, 1980;Dishman, 1982;Dishman & Sallis, 1994;Oldridge, 1988) Paterson, 1998;Oldridge, 1982). The available studies generally looked at dropout data for younger adults (i.e., <50 years old), and these might not be representative for older populations (King, Blair, Bild, & Dishman, 1992). Some researchers have suggested that older adults might exercise more consistently because they are more concerned about their health and have more time to exercise than younger adults do (Lee et al., 1996;Oldridge, 1988). In contrast, other researchers have shown that, in community exercise programs, older people are no more compliant than younger people (Ecclestone et al.). Older people might also be more likely to interrupt their exercise programs because the incidence of chronic diseases increases with advancing age. Obstacles to habitual exercise can lead to lapses, defi ned as slight periods of nonparticipation in the exercise program, and ultimately to dropout. In our study, we have defi ned dropout in terms of stopping with a specifi c exercise program, rather than a cessation of all physical activity. To date, only one study (Ecclestone et al., 1998) investigated the behavior of participants in switching from one exercise program to ano...