Twenty subjects spent a mean of 78 min seated in an unventilated smoke-filled room of approximately 43 m3 (15 × 12 × 8 ft). The smoke was produced by burning and smoking eighty cigarettes and two cigars. The average ambient carbon monoxide (CO) concentration was 38 ppm. Blood samples taken before and after the exposure showed an increase in carboxyhaemoglobin (COHb) in all subjects without exception (P<0·001). The COHb of the twelve non-smokers increased from a mean of 1·6% to 2·6% (P<0·001), while the six cigarette smokers, all inhalers, besides having significantly higher initial levels (P<0·001) also showed a greater increase from a mean of 5·9% to 9·6% (P<0·001). The two cigar smokers, one an inhaler, the other a non-inhaler, showed respective COHb changes similar to the cigarette smokers and non-smokers. The mean increase of 1% COHb among the non-smokers was similar to the mean increase for the smokers of 0·7% for each cigarette smoked, suggesting that the amount of CO which the non-smokers absorbed by passive smoking was about the same as would be expected if they had actively smoked and inhaled one cigarette.