Glucocorticoids seem to mediate the effect of stimulant drugs such as nicotine. Several studies have pointed to an association between the BclI polymorphism in the glucocorticoid receptor gene and increased glucocorticoid effects. We analysed the association of smoking behaviour and the BclI polymorphism using a case–control design within the framework of a larger pharmacogenetic study. A total of 327 Caucasian patients with asthma or chronic obstructive pulmonary disease from 39 German general practices gave informed consent to take part in the study. They filled in questionnaires concerning their smoking behaviour and were genotyped for the BclI polymorphism. The genotype frequencies for non-smokers (n = 251; CC, 0.42; CG, 0.46; GG, 0.12) as well as for smokers (n = 76; CC, 0.29; CG, 0.55; GG, 0.16) were consistent with the Hardy–Weinberg equilibrium. The proportion of smokers was significantly lower among carriers of the CC-genotype (22/127 = 17%) compared with carriers of the G-allele (54/200 = 27%; χ2 = 4.08; P = 0.04). Within the group of smokers, the proportion of heavy smokers (> 19 cigarettes/day; median) was reduced in C-homozygous patients when compared with carriers of the G-allele (7/22 = 32% versus 31/54 = 57%; χ2 = 4.09; P = 0.04). Stepwise logistic regression analysis also pointed to an association between the CC-genotype and a reduced probability of being a smoker (odds ratio = 0.55; 95% confidence interval = 0.30–1.00; P = 0.05) controlling for other predictors. In summary, this study provides evidence that the BclI polymorphism might play a role in the maintenance and severity of nicotine dependence.