I used estimates of energetic requirements of pumas (Puma concolor) to estimate prey requirements for mule deer (Odocoileus hemionus). This estimate might help explain the impact pumas may have on deer populations. Accepted daily energy demands of pumas result in estimates of annual kill rates per puma of 44 to >100 mule deer/yr. However, recent studies of puma activity indicate that the methods used to calculate these earlier values may have been flawed. To test this possibility, I used a more recently developed allometric equation to calculate energy demand of pumas based on movement data from a radio telemetry study in south-central Idaho and northwestern Utah. I calculated energy demand for 20 adult female and 8 adult male pumas based on their movements during 95 monitoring blocks of 24 hrs. I then calculated prey requirements for individuals and for low and high population estimates from the study area. I estimated that daily energy demands for male pumas (3,143.7 ± 120.1 kcal; 13,203.5 kilojoules; kJ) and female pumas with kittens (2705.4 ± 57.0 kcal; 11,362.7 kJ) were substantially lower than previously reported estimates of approximately 5,500 kcal/day (23,100 kJ/day) for males and females with an average (2.6 kittens) litter size. The resulting estimated annual kill rates (19.4 deer/yr for males, 39.6 deer/yr for females with kittens) were also substantially lower than previous estimates (44 deer/yr and 73 deer/yr respectively). I demonstrated that the earlier energy demand estimates were based on overestimations of puma activity for these 2 social groups and that my estimates of activity, and thus energy demand, are more biologically reasonable for pumas. I discuss the need for a reassessment of the impact pumas can have on deer populations in light of these new energy calculations.