We examined the ability of the Tritrac-R3D to estimate daily energy expenditure (EE) and characterize the physical activity patterns of free-living humans. Daily EE was estimated by the Tritrac, a 3-d physical activity log (PAL), and a 7-d recall (SDR). Digital storage of activity data by the Tritrac allowed us to tabulate the number of minutes the monitor recorded EE at specific activity intensities. Twenty-five men and women (mean age and body mass 26.7 yr, 72.5 kg) wore the monitor for 7 d. The Tritrac significantly underestimated daily EE compared with the PAL and SDR, 2552.7 vs 2915.5 kcal.d-1 and 2530.0 vs 2840.3 kcal.d-1, respectively (both, P < 0.01). Correlations between the Tritrac and PAL and the SDR were r = 0.82 and r = 0.77 (both, P < 0.001). Compared with the PAL, the Tritrac overestimated time accumulated in sedentary activities (862.7 vs 827.2 min.d-1) but underestimated time accumulated in active behaviors (37.5 vs 78.0 min.d-1). In conclusion, these data suggest the Tritrac ranks activity levels similarly to the PAL and SDR but it significantly underestimates free-living energy expenditure.