Self-adhesive patches containing a 7-day supply of transdermal clonidine were used to treat twenty patients with mild essential hypertension. These skin patches (3.5 cm2), which were changed by the patients every week, reduced diastolic blood-pressure to less than 90 mm Hg in twelve patients. When placebo-containing patches were substituted in these twelve patients after 3 months of treatment, blood-pressure rose slowly to its pretreatment level. Side effects appeared to be milder than those experienced during conventional oral antihypertensive treatment. Plasma clonidine concentrations were lower than peak levels after oral administration. This new method for treating hypertension was convenient and well tolerated, and may increase patient compliance.