The fact that exhaled breath contains valuable information on the health status of a subject has been known since the time of the ancient Greeks (eg, fetor hepaticus). Breath analysis offers a unique opportunity to retrieve relevant information on ongoing internal biochemical processes noninvasively, as parts of the most volatile components of blood reach the gas phase and are subsequently exhaled (eg, detection of ethanol in breath). In addition, because the respiratory tract is in direct contact with air, it also contributes to the composition of exhaled breath. For example, simple methods of breath analysis have been used in the assessment of bronchial inflammation by measuring nitric oxide. For these reasons, and as shown by a substantial number of investigations published on this topic, analysis of breath may be a valuable tool in diagnosing not only lung diseases but also other conditions that disturb metabolism, such as diabetes and renal or liver failure. However, despite this appealing approach, breath analysis is still in its infancy.1 Compared with the clinical analysis of other body fluids and tissue specimens, exhaled breath is still a far way from reaching the maturity needed to widely support clinicians, but major steps forward have been made during the past decade in an attempt to improve this situation. For example, major technological developments have enabled the detection of a wide range of analytes in breath, and importantly, this can be achieved in real time.2,3 Relevant metabolites such as isoprene,2,3 methanol,2,3 acetone,2,3 urea,4 free fatty acids,5 and a number of yet unidentified compounds have been reported to be detectable in human breath.6Figure 1 schematically shows a subject breathing into a mass spectrometer; in only 3 min, three replicate measurements with excellent repeatability provide breath mass spectrometric fingerprints. In addition, modern mass spectrometry not only is suitable for breathprinting but also allows a structural elucidation of the compounds detected in breath. For example, the trace shown in Figure 1 illustrates the breath-to-breath (three replicated measurements) detection of indole (tryptophan metabolism, C8H7N, molecular structure displayed) in human exhaled breath.