An instrument that accurately measures oxygen consumption (VO2) during field performance is valuable for investigations of physiological workload. Cosmed (Rome, Italy) has introduced such an instrument, the Cosmed K2. In this study the Cosmed K2 was compared with the Oxyconbeta (Jaeger, Breda, The Netherlands) to investigate the reliability of the ventilatory data obtained with the Cosmed K2. The measured values of the Cosmed K2 ranged from 780 to 3700 ml min(-1). When measuring light to heavy work situations (VO2 < 1400 ml min(-1)), the Cosmed K2 did not reproduce very well. This random error prohibits accurate measurements on individuals for those workloads. In very heavy to extremely heavy work situations (VO2 > 1500 ml min(-1)) the Cosmed K2 measured oxygen consumption values that were too low. This systematic error can be corrected. A correction relation was calculated by comparing the results of the Cosmed K2 with those of the Oxyconbeta in a laboratory setting. In the field of ergonomics, where mean values of workloads have to be determined, the Cosmed K2 can be used. In the field of sports, where individual improvement is important, the Cosmed K2 can be useful to measure oxygen consumption at moderate and high strain levels (VO2 > 1500 ml min(-1)).