Plasma potassium is a moderately heritable phenotype, but no robust associations between common single nucleotide polymorphisms (SNPs) and plasma potassium have previously been described. Genetic influences on renal potassium handling could be important in the aetiology of hypertension. We have tested whether common genetic variation in the gene encoding the beta subunit of the epithelial sodium channel (SCNN1B) affects plasma potassium and blood pressure level in a study of 1425 members of 248 families ascertained on a proband with hypertension. We characterised family members for blood pressure using ambulatory monitoring, measured plasma potassium in venous blood samples, and genotyped four SNPs which spanned the SCNN1B gene. We found highly significant association between genotype at the SCNN1B rs889299 SNP situated in intron 4 of the gene and plasma potassium. Homozygotes for the rarer T allele had on average a 0.15mM lower plasma potassium than homozygotes for the common C allele, with an intermediate value for heterozygotes (trend p=0.0003). Genotype at rs889299 accounted for approximately 1% of the total variability in plasma potassium, or around 3% of the total heritable fraction. There was no association between genotype at any SCNN1B SNP and blood pressure considered as a quantitative trait, or with hypertension affection status. We have shown a modest sized but highly significant effect of common genetic variation in the SCNN1B gene on plasma potassium. Interaction between the rs889299 SNP and functional SNPs in other genes influencing aldosterone-responsive distal tubular electrolyte transport may be important in the aetiology of essential hypertension.