The clonidine transdermal therapeutic system (TTS) is a cutaneous delivery device which provides therapeutically effective doses of clonidine at a constant rate over 7 days. In clinical trials it reduces blood pressure in patients with mild to moderate hypertension as effectively as oral clonidine but with greater stability of blood pressure control. Most patients find the transdermal system more convenient than oral treatments, and compliance may be improved. The side effects known to occur with orally administered clonidine, dry mouth and sedation in particular, are also produced with transdermal administration, but possibly at a lower incidence than during oral treatment. A proportion of patients experience adverse skin reactions with the transdermal system. At this stage of its development, transdermal clonidine has not been adequately compared with other 'standard' antihypertensive treatments such as diuretics or beta-adrenoceptor blocking drugs. However, despite the lack of such comparative studies, transdermal clonidine represents a worthwhile new approach to antihypertensive therapy, particularly in terms of patient convenience.