The authors evaluated the minute ventilation/carbon dioxide production relation (VE/VCO2 slope) as a complementary measure to peak oxygen consumption (peak VO2) in 76 patients (mean +/- SD age = 44.3+/-10.8 years, 69.7% female) with morbid obesity (mean +/- SD body mass index [BMI] = 49.4+/-7.0 kg/m(2)), as it is not limited by effort. Nearly one-half (43%) of the patients achieved a peak respiratory exchange ratio <1.10. Mean peak VO2 and VE/VCO2 slope were 17.0+/-3.7 mL/kg/min and 27.8+/-4.0, respectively. Peak VO2 correlated with BMI (r=-0.45, P<.0001), while VE/VCO2 slope did not (r=-0.04, P=.73). There was a linear trend for declining mean peak VO2 (P=.001) but not for VE /VCO2 slope (P=.59) with increasing BMI quintiles. The VE/VCO2 slope is an effort-independent measure that is also independent of BMI and may serve as an adjunctive cardiorespiratory variable when evaluating morbidly obese men and women.