Inhalation of small amounts of carbon monoxide diminishes the pain threshold in patients with stable angina pectoris. The aim of this study was to identify and describe patients who had been exposed unknowingly to toxic inhalations of this gas and subsequently presented to hospital with a clinical picture of unstable angina. Blood carboxyhaemoglobin levels of 104 patients referred with unstable angina to a coronary care unit were determined on admission. The likely source of carbon monoxide was identified in all patients. Three patients had definite carbon monoxide intoxication. Another five patients had evidence of minor exposure. When the three cases with carbon monoxide poisoning were excluded, the mean carboxyhaemoglobin level was 2.5% (+/- 1.3) for smokers (n = 30) and 0.6% (+/- 0.5) for non-smokers (n = 71). Use of fossil fuel combustion in an enclosed environment was responsible for the three most serious intoxications and one of the minor cases. We suggest that a number of patients admitted for coronary care with unstable angina may have significant carbon monoxide poisoning. This intoxication is often overlooked by attending physicians with the result that high concentration oxygen therapy is not administered, when it is in fact a necessary part of treatment.