The reliability of self-reported smoking behaviour can vary and may result in bias if errors in misclassification vary with outcome. We examined whether self-report was an accurate measure of current smoking status in patients with malignant or non-malignant respiratory disease. Smoking behaviour was assessed by self-report and by analysis of whole blood for cotinine, a biomarker of exposure to cigarette smoke, in 166 patients attending a bronchoscopy clinic. Cotinine levels ranged from 2.5 to >400 ng ml(-1) blood and were higher in self-reported current smokers (173+/-123 ng ml(-1)) than in never smokers (3.7+/-8.7 ng ml(-1)) or ex-smokers (20.5+/-49.0 ng ml(-1)). Cotinine levels in self-reported current smokers increased with the numbers of cigarettes smoked (p=0.06), and levels in smokers and ex-smokers decreased with the reported length of time since the last cigarette (p=0.001). Using a cotinine level of 20 ng ml(-1) and self-report as the gold standard, the sensitivity and specificity for defining current smoking status were 90.2% and 82.4%, respectively. Out of a total of 125 self-reported current non-smokers, 23 (18.4%) had cotinine levels greater than 20 ng ml(-1). Smoking prevalence was significantly underestimated by self-report (24.7%) when compared with that defined using blood cotinine levels (36.1%: p<0.001). Misclassification of current smoking status was particularly high in ex-smokers, in patients without malignant respiratory disease, in men, and in those below the median age. Such differential misclassification may result in bias in studies examining associations between current smoking habits and disease risk.