Young adults are often in periods of transition, and lifestyle changes such as a decline in physical activity can occur during this period. The purpose of this study was to determine the relationships between demographic, biologic, lifestyle, social support index, environmental factors, and physical activity in young adults. The participants were 4,152 young adults from 18 to 30 years of age enrolled in the Third National Health and Nutrition Examination Survey (NHANES III). A moderate to vigorous physical activity (MVPA) score was calculated from responses to nine activities and up to four activities not previously listed. Multiple regression analyses were used to assess the relationship between MVPA and independent variables separately for men and women. Non-Hispanic Blacks were more active than non-Hispanic White or Mexican American men, whereas, among women, non-Hispanic Whites were more active. Education, social support index, and trying to lose weight were positively associated with MVPA, whereas being married was inversely related in both men and women. Among women, those who were unemployed, in better health, had smaller families, had lower body mass indexes (BMIs), and were not from the South had higher MVPA. Men who were in school during the past 12 months were more active than those who were not in school. The results from this study suggest that demographic and social factors are important determinants of physical activity in young adults and should be considered when planning interventions.