Intial cardiorespiratory complaints in an emergency room were studied to determine their relationship to ambient levels of carbon monoxide recorded at a nearby permanent station. All 8,556 encounters with patients (daily mean, 93) were studied during Denver's peak season for carbon monoxide pollution (three-month winter period). The one-hour mean maximum level of carbon monoxide was above 17.8 ppm, averaging 27.2 +/- 4.3 ppm (vs 12.1 +/- 1.8 ppm on "low carbon monoxide days," P less than 0.0001); and when significantly higher frequencies of cardiorespiratory complaints occurred (7.9 +/- 0.7 percent vs 6.4 +/- 0.3 percent [+/- 1 SE]; P less than 0.04), the mean 24-hour level of carbon monoxide averaged 9.3 +/- 1.4 ppm (vs 5.9 +/- 0.8 ppm on "low carbon monoxide rays;" P less than 0.001). The frequency of cardiorespiratory complaints also was correlated with each measurement of the ambient level of carbon monoxide (one-hour mean maximum level, 24-hour mean level, and two-day moving averages of each) in six of eight comparisons. These observations strongly suggest that the frequency of thoracic complaints in an emergency room can be affected by the ambient level of carbon monoxide.