The clonidine transdermal therapeutic system is being used as a therapy for blood pressure treatment. Systemic side effects seem to be fewer than with oral clonidine. However, localized skin reactions occur frequently and the incidence increases with the dose and duration of use. Common signs include erythema, scaling, vesiculation, excoriation, and induration. Allergic contact dermatitis is less frequent but common. Hyperpigmentation and depigmentation also occur. Pretreatment with 0.5% hydrocortisone is associated with less skin irritation and higher blood levels. Although oral clonidine is no longer widely used in the management of hypertension, awareness of these reactions to the transdermal delivery of this agent is important.