Background. This study investigated the relationships among optimism, pessimism, physical activity, and dietary behaviors among 8- to 10-year-old African-American girls in the Girls' Health Enrichment Multisite Studies (GEMS). Methods. Ninety-two girls were randomly assigned to a 12-week physical activity and diet intervention or comparison group and completed psychosocial assessments including the Youth Life Orientation Test (YLOT) of optimism and pessimism, physical performance self-concept, physical activity self-efficacy, physical activity outcome expectancies, and physical activity and sedentary preferences. Also, preferences for bottled water and sweetened beverages were assessed. Physical activity and dietary intake were assessed by self-report and parent-report at baseline and follow-up. Also, physical activity was objectively assessed by wearing an accelerometer for 3 days. The psychometric properties of the optimism-pessimism subscales were analyzed. Results. The measures of optimism and pessimism in children were reliable (r = 0.75-0.82). In the multiple regression analyses without the intervention interaction terms, pessimism was positively and significantly related to increases in MET-adjusted usual activity (P = 0.008) and sedentary behaviors (P = 0.0004). Additionally, a negative (P = 0.026) pessimism by intervention interaction term for MET-adjusted usual activity was found such that the intervention group had a lower change in physical activity per unit increase in pessimism compared to the control group. Conclusions: Among 8- to 10-year-old African-American girls, pessimism was related to increased sedentary behaviors and usual activity. Previous studies have reported relationships between optimism and health-compromising behaviors. This study found that pessimism may positively or negatively influence efforts to increase health-promoting behaviors. Future research should confirm and clarify the meaning of these findings.