The purpose of this study was to determine the validity and evaluate the accuracy of a portable telemetric oxygen uptake analyzer (K2). Two experiments were carried out: a) using a mechanical lung, the accuracy of the K2 to measure oxygen fractions and minute ventilation following 10 and 60 min of warm-up was determined; and b) two maximal graded exercise tests (GXT) on 15 subjects, one with the K2 system and the other with a standardized breath-by-breath (BBB) system, while heart rate (HR), minute ventilation (VE), and oxygen uptake (VO2) were compared. Following 10-min warm-up prior to calibration, the K2 underestimated the true oxygen fraction as early as 5 min into the test, and this value continued to decrease throughout the 30-min test. After 60 min of warm-up prior to calibration, the K2 accurately measured the true oxygen fraction for the first 15 min; at minute 20, and on to minute 30, the K2 underestimated the oxygen fraction. Ventilation volumes were not affected by warm-up time. Minute ventilation during the K2 GXT was significantly higher than VE for the BBB test. No significant differences were found between the HRs obtained with the BBB or K2 systems. No differences in VO2 for any stage of the GXT were identified between the K2 device, BBB device or when a respiratory exchange ratio (RER) correction factor was applied to the K2 derived values. However, the RER correction factor did minimize the VO2 differences between the BBB and K2 systems. Therefore, we conclude that the K2 accurately measures VO2 during a GXT; however, its accuracy can be compromised by limitations inherent to the system.