The Caltrac accelerometer is designed to assess energy expenditure during normal activity. The aims of the present study were (a) to determine the interinstrument and intersession reliability of the Caltrac and (b) to validate these instruments with measures of oxygen uptake (VO2) and determine whether Caltracs accurately reflect differences in energy expenditure due to sex, body weight, and speed. Caltracs were found to have very good interinstrument and intersession reliability and to accurately reflect differences in expenditure due to sex, weight, and speed. However, Caltracs consistently over-estimated energy expenditure by an average of 9-13%. The absolute percent error (percent error without consideration of the sign) of the Caltrac was greater, averaging 23%. The correlations between Caltrac and VO2 averaged 0.68-0.74. Thus, the Caltracs seem well suited for studies of activity level of groups, but may be less useful in estimating an individual's activity.