Work capacity and cardiopulmonary performance were studied in a group of 11 young obese subjects (BMI 39.9 kg/m2) and a group of 10 young normal subjects (BMI 22 kg/m2). First of all they underwent an incremental cycle ergometer test up to exhaustion. Subsequently, every subject of the two groups performed a constant work rate test at different work loads to estimate cardiac output (Q) below anaerobic threshold (AT) by a 20-second CO2 rebreathing method. Obese subjects had a significantly lower AT (79 vs. 109 W). The ratio between oxygen uptake and heart rate (VO2/HR) (O2 pulse) was higher in the obese group; nevertheless, this variable became significantly lower if we took into consideration the ratio between O2 pulse and kilogram fat-free body mass or kilogram body weight. Both these observations suggest that their reduced work tolerance is linked with a reduced oxygen supply to the muscles in activity. Q increased in similar ways in obese and normal subjects at the preset work rates. The ratio Q/body surface (cardiac index; CI) that we considered in order to try to minimize the differences in body sizes between the two groups, increased less in response to increasing work rates in our obese subjects than in normal subjects. As a whole, these data appear to be in line with a relatively less efficient cardiac performance during progressive work rates in obese subjects.