A new method was developed to assess environmental tobacco smoke in air. The method is based on passive sampling and subsequent measurement of the concentration of 3-ethenylpyridine, a vapor-phase compound specific to tobacco smoke. Air samples were collected using a 3M organic vapor monitor. Tests were carried out in a dynamic chamber to determine the sampling rate (25.7 cm3/min). 3-Ethenylpyridine was desorbed from the sampler with 1 mL of pyridine/toluene mixture. 3-Ethenylpyridine was quantified by gas chromatography/mass spectrometry. The limit of detection was 0.01 microgram/sample, corresponding to a concentration of 0.27 microgram/m3 air calculated for a sampling period of 24 h. Field measurements were carried out to test the performance of the method. Mean concentrations ranging from 1.3 to 5.3 micrograms/m3 were measured for 3-ethenylpyridine in smoking environments, but no 3-ethenylpyridine was detected in nonsmoking environments. Active sampling using charcoal tubes was used as a reference method in the chamber tests and field measurements. Individual exposures can be easily and accurately measured by means of the passive sampler. Because of simple sample treatment, the method is also well-suited for large-scale monitoring of environmental tobacco smoke.