HAYMES, E. M. and W. C. BYRNES. Walking and running energy expenditure estimated by Caltrac and indirect calorimetry. Med. Sci. Sports Exerc., Vol. 25, No. 12, pp. 1365–1369, 1993. The purpose of this study was to examine the accuracy of the Caltrac personal activity computer during walking and running. Ten women and 10 men walked at speeds of 2–5 mph and ran at speeds of 4–8 mph on a horizontal treadmill. Two Caltrac monitors were attached over opposite hips: one programed to give caloric expenditure and the other to give Caltrac counts. Oxygen uptake was measured simultaneously. Significant correlations were found during walking between Caltrac estimated and actual energy expenditure (r = 0.91) and between activity counts and net exercise JOURNAL/mespex/04.02/00005768-199312000-00008/ENTITY_OV0312/v/2017-07-20T222403Z/r/image-pngO2.kg-1 (r = 0.87). However, the Caltrac significantly overestimated energy cost during horizontal walking at speeds above 2 mph. Although there was a significant correlation between Caltrac estimated and actual energy expenditure during running (r = 0.71), the correlation between Caltrac counts and net exercise JOURNAL/mespex/04.02/00005768-199312000-00008/ENTITY_OV0312/v/2017-07-20T222403Z/r/image-pngO2.kg-1 was not significant (r = 0.29). There was no significant increase in Caltrac kcal or counts with increased running speed between 5 and 8 mph. It is concluded that the Caltrac is a valid indicator of physical activity during walking but does not adequately discriminate between running speeds of 5–8 mph.