An epidemiologic and clinical investigation of 184 persons exposed to toxic levels of carbon monoxide (CO) in a public high school has been carried out. Exposure to 500 ppm of CO for periods up to 150 minutes resulted in carboxyhemoglobin (COHb)levels as high as 30% and symptomatic illness in 87% of persons exposed before the possibility of mass CO poisoning was recognized. Severity of symptoms was a reliable basis for triage of victims for immediate hospital treatment, and correlated both with duration of exposure and the length of time it took for victims to feel normal. Therapy consisting of oxygen delivered by mask achieved a half-life for COHb of 137 minutes in hospital-treated patients (compared to 320 minutes with no treatment), but did not fully reverse the symptoms of CO toxicity.