The attractiveness of mass spectrometer technology comes from its fundamental capability of measuring a large number of gas constituents simultaneously with precision and high speed. During the past two and one-half decades the mass spectrometer has proved to be extremely useful in the laboratory. In this environment the technology has provided the opportunity for innovative research and new discoveries in respiratory physiology. Improved technology (such as new and innovative engineering in vacuum technology, electronics, and microprocessors) has now made this analytic technique practical in the clinical environment. It can be expected that these instruments will continue to play an important role in the future in patient monitoring systems. Because of the specificity and sensitivity in the mass spectrometer, its role in noninvasive monitoring is expected to be most significant. Methods for noninvasive measurements of important physiologic parameters such as cardiac output and lung functions are well established in the research laboratory. These methods depend on the use of small amounts of trace gases, many of which can be effectively measured only by a mass spectrometer. Since the mass spectrometer has been the tool for developing these techniques in the laboratory, it will also be the focus of future noninvasive cardiopulmonary monitoring systems used routinely in the clinical environment.