In clinical practice, oxygen uptake is an indicator of cardiopulmonary performance. Most commercial systems measure oxygen uptake by collecting expired air through masks or mouthpieces which are often poorly tolerated by the patient. We have developed and validated a novel mask-free system to improve patient comfort and performance. The prototype is composed of a soft walled funnel that collects and conveys the expired air, together with some external air, in a mixing chamber by means of an aspiration system. Oxygen concentration and airflow are measured and then oxygen uptake is calculated. Direct comparison between calculated and preset oxygen uptake values obtained by a mechanical simulator was performed. Errors ranged between 1% and 3.3%, depending on the absolute value of oxygen consumption. Then the prototype was connected "in-series" with a breath-by-breath commercial system, and ten subjects were submitted to a standard stress test. The results showed good agreement (R=0.94) and a mean difference of 5% between the peak values. The longer response time of the prototype caused a delay between the two .V(O2)(t) curves, leading to an underestimation in the exercise phase and an overestimation in the recovery phase, suggesting more technical improvements. Nevertheless in its present form the new system can be used in the whole exercise phase and, with caution, also in the recovery phase.