1775 subjects were asked about their current use of tobacco products or nicotine chewing gum. 1537 provided a sample of saliva for cotinine analysis. Of 808 who claimed not to be users of such products, 20 (2.5%) had cotinine values above 30 ng/ml, suggesting their self-reports were false. In another study, 540 subjects were interviewed on two occasions. 10% of subjects claiming on one occasion never to have smoked made inconsistent statements on the other occasion. A third study showed a strong tendency for smokers to marry smokers. Bias caused by misclassification of smoking habits coupled with between-spouse smoking habit concordance can completely explain reported apparent excesses in lung cancer risk in non-smokers married to smokers.