Wrist-worn fitness trackers are popular consumer products, but few studies have documented their use as a stand-alone intervention tool. This study examined the effect of wearing the Fitbit Flex on moderate to vigorous physical activity (MVPA), sedentary behavior (SB), self-efficacy (SE), and cardiometabolic risk factors. Participants (n=28, m age = 50.5 yrs) wore the Fitbit for 3 weeks, then were randomly assigned to continue for 3 more weeks (6week group) or discontinue use (3-week group). Participants were not physically active prior to the study. Accelerometers recorded weekly, work-week and weekend MVPA and SB at baseline and post. BMI, waist circumference, and blood pressure were measured baseline and post. A self-efficacy questionnaire was administered baseline, mid-point, and post. MVPA significantly increased in both groups. The 6-week group performed better on weekend MVPA. SB did not change. Waist circumference decreased significantly in both groups. Overall, there was no change in SE. However, the hypothesized pattern between groups was found. The 6-week group had a small increase in SE while the 3-week group declined non-significantly. Fitbit fitness trackers are effective at increasing MVPA and impact waist circumference, in as little as 6 weeks. However, the mechanism for behavior change is unclear and needs further study. This research is challenging as the features of fitness trackers are constantly changing.