Breath testing for volatile organic compounds (VOCs) provides an intrinsically safe method for investigating human metabolism. An improved breath-collecting apparatus (BCA) is described which was acceptable to patients, simple to use, highly sensitive, and free from chemical contamination. VOCs in 10.0 L alveolar breath and 10.0 L room air were collected onto adsorptive traps. Using automated instrumentation, VOCs were thermally desorbed and assayed by gas chromatography/mass spectroscopy. Twenty normal volunteers were studied, and the alveolar gradient (concentration in breath minus concentration in air) was determined for the most abundant VOCs. A total of 1259 VOCs were observed and tentatively identified in the breath of normal subjects. The mean alveolar gradients were positive in 461 VOCs and negative in 798 VOCs. The method provided a sensitive and convenient assay for breath VOCs and permitted tentative determination of their origin from either inside or outside the body.