High blood pressure in children is usually the result of defined disorders of the kidney and hormonal systems. Renal artery stenosis (RAS) describes a critical narrowing of renal arteries, which supply blood to the kidneys. It is most commonly associated with a malformation of the main kidney artery, called ‘fibromuscular dysplasia’ (FMD). The affected kidneys make excess hormones, which increase blood pressure causing potentially fatal effects on diverse organs. RAS/FMD is a disorder of arterial wall maturation and it can run in families.  This study will focus on thirty individuals affected by RAS/FMD who have been cared for at Great Ormond Street Hospital to establish a unique genetic resource of DNA from affected UK families, acting as a prelude to further work to define mutations.