Differences in glucocorticoid (GC) sensitivity may underlie both common diseases (e.g. hypertension) and variability in response to treatment with GCs (e.g. asthma). We tested the potential involvement of the GC receptor (GR) gene in mediating GC sensitivity using haplotype analysis and a low-dose dexamethasone suppression test. Linkage disequilibrium across the GR gene was determined in 216 U.K. Caucasians, and 116 had a 0.25-mg overnight dexamethasone suppression test. Very strong linkage disequilibrium was observed across the GR gene with only four haplotypes accounting for 95% of those observed. Haplotype pattern mining and linear regression analyses independently identified a three-marker haplotype, across intron B, to be significantly associated with low postdexamethasone cortisol (P = 0.03). Carriage of this haplotype occurred in 41% of the individuals with low postdexamethasone cortisol vs. 23% in the combined other quartiles (odds ratio 2.4, 95% confidence interval 0.9-6.3, P = 0.05). This is the first comprehensive, haplotype based analysis of the GR gene. A three-point haplotype, within intron B, is associated with enhanced sensitivity to GCs. This haplotype may help predetermine variation in clinical response to GC therapy and also assist the understanding of diseases related to GC production.