The aim of this population based, nested, case-control study was to determine the risk of stroke associated with oral and transdermal administration of hormone replacement therapy. The study matched 15,710 cases of stroke to 59,958 controls. The types of HRT were categorised into estrogens only, estrogens plus progestogens, progestogen only and tibolone. These treatments were further subcategorised according to delivery method (oral versus transdermal) and dose (high versus low). Overall, the study found that there was no increase in the risk of stroke with the use of low estrogen dose patches compared to no use. However, there was a significant increase in risk associated with high dose transdermal estrogen patches. Users of both low and high dose oral HRT had a significantly increased risk of stroke than non-users.