Purpose: This study tested the hypothesis that volunteering in environmental organizations in midlife is associated with greater physical activity and improved mental and physical health over a 20-year period. Design and Methods: The study used data from two waves (1974 and 1994) of the Alameda County Study, a longitudinal study of health and mortality that has followed a cohort of 6,928 adults since 1965. Using logistic and multiple regression models, we examined the prospective association between environmental and other volunteerism and three outcomes (physical activity, self-reported health, and depression), with 1974 volunteerism predicting 1994 outcomes, controlling for a number of relevant covariates. Results: Midlife environmental volunteering was significantly associated with physical activity, self-reported health, and depressive symptoms. Implications: This population-based study offers the first epidemiological evidence for a significant positive relationship between environmental volunteering and health and well-being outcomes. Further research, including intervention studies, is needed to confirm and shed additional light on these initial findings.