In calculating the physical activity index (PAI) on the college alumnus questionnaire, it is assumed that 8 kcal are expended for every 20 steps climbed. This value is equal to an energy cost of 0.40 kcal.step-1. Since it is assumed that subjects climb and descend an equal number of stairs, the total value reflects the energy cost of stepping up (estimated at 0.30 kcal.step-1) and stepping down (estimated at 0.10 kcal.step-1). However, these values appear to be based on theoretical calculations rather than empirical observation. The purpose of this study was to quantify the energy cost of stair climbing and stair descending by measuring oxygen uptake. Twenty subjects performed continuous stair-climbing and stair-descending on an escalator at a stepping rate of 70 step.min-1. Heart rate was monitored by telemetry, and oxygen uptake was measured by the Douglas bag technique from 5 to 7 min. Results showed that the gross energy cost of stair climbing is 8.6 METs, and that of stair descending is 2.9 METs. Thus, for a 70-kg person the gross caloric costs of ascending stairs (0.15 kcal.step-1) and descending stairs (0.05 kcal.step-1) are one-half of the values previously assumed. In conclusion, the algorithm for calculating PAI on the college alumnus questionnaire should be modified to reflect a total cost of 0.20 kcal for going up and down one step. Even more precise estimates can be obtained by adjusting for body weight (going up and down one flight of stairs requires 1.63 MET.min).