Effects of different variables (amount, number of puffs, depth of inhalation, length and timing of sampling) of smoking on the blood carboxyhaemoglobin (COHb) levels were studied. There were 27 bidi smokers, 25 cigarette smokers, 25 mixed smokers (all asymptomatic) and 20 symptomatic cigarette smokers. Fifty-eight healthy non-smokers served as controls. The mean COHb levels in all the smokers were nearly 5-7 times higher than that in non-smokers. The blood COHb levels correlated well with the self assessed degree of inhalation by the individual smokers in each of the groups. There was no significant correlation between the COHb levels and number smoked on the test day, number of puffs per cigarette or bidi, and the length of smoking material. There was a good inverse correlation between the time interval and COHb level. The smoking index (amount smoked) did not bear any relationship with blood COHb levels.