The genotype at the C-11377G single-nucleotide polymorphism (SNP) (rs266729) in the adiponectin gene promoter has been shown to affect the prevalence of coronary atherosclerosis and incidence of vascular events in men, and to affect carotid intima media thickness. We have examined the relationship between this polymorphism and blood pressure in a cohort ascertained to express variability in blood pressure measurements. We studied a cohort of 255 families comprising 1425 individuals ascertained via a hypertensive proband. Blood pressure was measured by ambulatory monitoring. The C-11377G SNP was genotyped using a TaqMan assay. There was evidence of association between this SNP and log systolic blood pressure (SBP), having adjusted for significant covariates including gender, age and drug treatment; P=0.009, 0.014 and 0.022, respectively, for daytime, night-time and clinic measurements. Replacing C by G caused an increase of 1.63, 1.83 and 1.61%, respectively, per gene copy. There were smaller effects on diastolic blood pressure and waist-hip ratio, which were of borderline significance. Genotype at the C-11377G (rs266729) polymorphism has independent effects both on waist-hip ratio and SBP. This may help in understanding the complex role that the adiponectin gene has in atherosclerosis.