Twenty male and female subjects were exposed repetitively to methylene chloride (CHCl) vapor, 50, 100, 250, and 500 ppm, for 1,3, or 7.5 h in a controlled-environment chamber. Postexposure alveolar breath samples were collected in small glass breath tubes and analyzed for CHCl by gas chromatography. From these data is a series of breath CHCl excretion curves were constructed that can be used to estimate the magnitude of a recent exposure. The CHCl breath concentration in the immediate postexposure period accurately reflected the vapor concentration to which the subjects had been exposed most recently. Breath samples collected 1-2 h following exposure were accurate indicators of the time-weighted average vapor exposure experienced by the subjects during the previous 8 h of occupational exposure. Breath analysis offers a practical, noninvasive method for monitoring occupational exposure to CHCl.