We compared cardiorespiratory responses to exercise on an underwater treadmill (UTM) and land treadmill (LTM) and derived an equation to estimate oxygen consumption (VO[subscript 2]) during UTM exercise. Fifty-five men and women completed one LTM and five UTM exercise sessions on separate days. The UTM sessions consisted of chest-deep immersion, with 0, 25, 50, 75, and 100% water-jet resistance. All session treadmill velocities increased every 3 min from 53.6 to 187.8 m[middle dot]min[superscript -1]. Cardiorespiratory responses were similar between LTM and UTM when jet resistance for UTM was [greater than or equal to] 50%. Using multiple regression analysis, weight-relative VO[subscript 2] could be estimated as: VO[subscript 2] (mLO[subscript 2][middle dot]kg[superscript -1][middle dot]min[superscript -1]) = 0.19248 [middle dot] height (cm) + 0.17422 [middle dot] jet resistance (% max) + 0.14092 [middle dot] velocity (m[middle dot]min[superscript -1]) - 0.12794 [middle dot] weight (kg) - 27.82849, R[superscript 2] = 0.82. Our data indicate that similar LTM and UTM cardiorespiratory responses are achievable, and we provide a reasonable estimate of UTM VO[subscript 2]. (Contains 4 figures and 3 tables.)