For exercise prescription purposes, it is often assumed that % heart rate reserve (%HRR) provides equivalent intensities to %VO2max. However, a recent study from this laboratory demonstrated that during cycling exercise %HRR is not equivalent to %VO2max, but is instead equivalent to a percentage of the difference between resting and maximal VO2, i.e., % VO2reserve (%VO2R). The current study examined these relationships during treadmill exercise. Fifty adults performed Bruce protocol treadmill tests to exhaustion. For each subject, data obtained at rest, at the end of each stage, and at maximum were used to determine linear regressions of %HRR versus %VO2max, and of %HRR versus %VO2R. For %HRR versus %VO2max the mean intercept and slope were -6.1+/-0.7 and 1.10+/-0.01, respectively, with a mean r of 0.990+/-0.002. For %HRR versus %VO2R, the mean intercept and slope were 1.5+/-0.6 and 1.03+/-0.01, respectively, with a mean r of 0.990+/-0.002. Both regressions differed statistically from the line of identity (i.e., intercept of 0 and slope of 1). However, the regression of %HRR versus %VO2R was significantly closer (P < 0.001 ) to the line of identity than was the regression of %HRR versus %VO2max. We conclude that %HRR should be considered as an indicator of %VO2R, not %VO2max, when prescribing treadmill exercise, as was previously concluded for cycling exercise.