Objective: Sedentariness is associated with weight gain and obesity. A treadmill desk is the combination of a standing desk and a treadmill that allow employees to work while walking at low speed. Design and Methods: The hypothesis was that a 1-year intervention with treadmill desks is associated with an increase in employee daily physical activity (summation of all activity per minute) and a decrease in daily sedentary time (zero activity). Employees (n ¼ 36; 25 women, 11 men) with sedentary jobs (87 6 27 kg, BMI 29 6 7 kg/m 2 , n ¼ 10 Lean BMI < 25 kg/m 2 , n ¼ 15 Overweight 25 < BMI < 30 kg/m 2 , n ¼ 11 Obese BMI > 30 kg/m 2 ) volunteered to have their traditional desk replaced with a treadmill desk to promote physical activity for 1 year. Results: Daily physical activity (using accelerometers), work performance, body composition, and blood variables were measured at Baseline and 6 and 12 months after the treadmill desk intervention. Subjects who used the treadmill desk increased daily physical activity from baseline 3,353 6 1,802 activity units (AU)/day to, at 6 months, 4,460 6 2,376 AU/day (P < 0.001), and at 12 months, 4,205 6 2,238 AU/day (P < 0.001). Access to the treadmill desks was associated with significant decreases in daily sedentary time (zero activity) from at baseline 1,020 6 75 min/day to, at 6 months, 929 6 84 min/day (P < 0.001), and at 12 months, 978 6 95 min/day (P < 0.001). For the whole group, weight loss averaged 1.4 6 3.3 kg (P < 0.05). Weight loss for obese subjects was 2.3 6 3.5 kg (P < 0.03). Access to the treadmill desks was associated with increased daily physical activity compared to traditional chair-based desks; their deployment was not associated with altered performance. For the 36 participants, fat mass did not change significantly, however, those who lost weight (n ¼ 22) lost 3.4 6 5.4 kg (P < 0.001) of fat mass. Weight loss was greatest in people with obesity. Conclusions: Access to treadmill desks may improve the health of office workers without affecting work performance.