Obesity has become one of the greatest public health issues in America. BodyMedia promotes their SenseWear Armband physical activity monitor as a way to help with weight management. Purpose: to assess the accuracy of the SenseWear Armband Mini-Fly (SWA-MF) in adults of different BMI categories during rest, recovery, and two walking speeds. Methods: Forty-six participants were assigned to one of three BMI classifications: normal weight (n =15; 18.5-24.9 kg.m-2 [kilograms per meter squared]), overweight (n=17; 25-29.9 kg.m-2), or obese (n= 14; ≥30 kg.m-2). Height and weight were measured. Participants began the test with15 minutes of seated rest, then walked on a treadmill for 8 minutes at 50 m.min-1 [meters per minute], engaged in a seated recovery for a second 15 minute period, and then walked on a treadmill for 8 minutes at 75 m.min-1. During the test, participants wore the SWA-MF over their left triceps, and the ParvoMedics metabolic system was used to measure oxygen consumption. Calories per minute (kcal.min-1) [calories per minute] were used to quantify energy expenditure in both systems. Results: The SWA-MF error score was not affected by the participants’ BMI (p = 0.543.). The SWA-MF significantly underestimated measured energy expenditure during the resting condition by 0.21 kcal.min-1 (p \u3c 0.001) and during the recovery by 0.27 kcal.min-1 (p \u3c 0.001), but significantly overestimated measured energy expenditure during walking at 50 m.min-1 by 0.70 kcal.min-1 (p \u3c 0.001). The SWA-MF was not significantly different from measured energy expenditure during walking at 75 m.min-1 (p = 0.672) or over the duration of the total testing session (p = 0.913). Bland-Altman plots for energy expended during the total testing session showed mean biases between -0.09 and 0.09 kcal.min-1, and 95% prediction intervals between -1.16 and 1.20 kcal.min-1.Conclusion: The primary finding from this study is that the validity of the SWA-MF does not differ among BMI groups. Secondary findings support that the SWA-MF underestimates measured energy expenditure during seated resting and recovery periods and overestimates measured energy expenditure during brief periods of slow walking at 50 m.min-1