The purpose of this study was to assess the validity of data derived from the Kenz calorie counter during progressive, incremental treadmill exercise. Direct comparisons were made with calories calculated from on-line gas analysis. The subjects were 18 adults, 18 postadolescent children, and 24 preadolescent children. Linear regression (r 2 > .95) showed a progressive deviation away from a 1:1 relationship between Kenz data and V̇O2 data with increasing age of subject which remained when standardized to kcal · kg−1 body mass or kcal · m−2 · hour−1. The Kenz calorie counter, after applying an age group correction factor, can thus be used as a suitable analog for measured energy expenditure.