The purpose was to determine the energy expenditure during ultradistance trail running. A portable metabolic unit was carried by a male subject for the first 64.5 km portion of the Western States 100 running race. Calibrations were done with known gases and volumes at ambient temperature, humidity and pressure (23-40.5 °C and 16-40% respectively). Altitude averaged 1692.8 ± 210 m during data collection. The male subject (36 yrs, 75 kg, VO₂max of 67.0 ml·kg^-1·min^-1) had an average (mean ± SD) heart rate of 132 ± 9 bpm, oxygen consumption of 34.0 ± 6.8 ml·kg^-1·min^-1, RER of 0.91 ± 0.04, and V_E of 86.0 ± 14.3 L·min^-1 during the 21.7 km measuring period. This represented an average of 51% VO₂max and 75% heart rate maximum. Energy expenditure was 12.6 ± 2.5 kcals·min^-1, or 82.7 ± 16.6 kcals·km^-1 (134 ± 27 kcals·mile^-1) at 68.3 ± 12.5% carbohydrate. Extrapolation of this data would result in an energy expenditure of >13,000 kcals for the 160 km race, and an exogenous carbohydrate requirement of >250 kcal·hr^-1. The energy cost of running for this subject on separate, noncompetitive occasions ranged from 64.9 ± 8.5 to 74.4 ± 5.5 kcals·km^-1 (105 ± 14 to 120 ± 9 kcals·mile^-1). Ultradistance trail running increases energy expenditure above that of running on nonundulating terrain, which may result in underestimating energy requirements during these events and subsequent undernourishment and suboptimal performance.