A prototype portable sensor for analyzing organic vapors in exhaled breath was described. The instrument consisted of an array of four polymer coated 250 megahertz surface acoustic wave (SAW) resonators mounted with matched reference sensors in a brass block. The block was in contact with a Peltier type temperature controller which maintained the sensors at 20 degrees-C. With this configuration, the working sensors were exposed to the environment to be sampled, while the reference sensors remained unexposed. In the presence of an analyte, the frequencies of the working and reference sensors were compared electronically using a radiofrequency mixer and the resulting difference frequency was measured using onboard digital counting circuits. The instrument was equipped with microprocessor controlled automated pumps, solenoid valves, and a nickel/chromium heating element which allowed samples to be preconcentrated and then thermally desorbed. Once a sample was collected, the sample was thermally desorbed from the preconcentrator, which allowed the sample to be drawn across the sensors. The microprocessor collected the output from the sensors and transmitted it to an external computer. The instrument was calibrated with perchloroethylene (127184), trichloroethylene (79016), and methoxyflurane (76380) at six concentrations ranging from 2 to 50 parts per million (ppm). Tenax TA was used as the preconcentrator adsorbent. The limits of detection were: perchloroethylene, 0.7ppm; trichloroethylene, 0.6ppm; and methoxyflurane, 4ppm. Breath samples, 250 milliliters, spiked with 2 to 18ppm of the three solvents collected in Tedlar bags were analyzed by the sensor. Parallel analyses were performed by gas chromatography for comparison purposes. The data obtained with the sensor were linearly correlated with the gas chromatographic data, correlation coefficient 0.97. The best agreement between the two data sets was obtained for perchloroethylene, the differences varying from -6 to 17%. The differences in the case of trichloroethylene and methoxyflurane varied from 2 to 39, and 2 to 54%, respectively. The authors conclude that sensors based on SAW technology are useful for analyzing organic vapors in breath samples under field conditions.