This study assessed the antihypertensive efficacy and side effects of clonidine administered transdermally. Twenty-five patients with mild to moderate essential hypertension (seated diastolic blood pressure 95-120 mm Hg with diuretic therapy alone) controlled with oral diuretic plus oral clonidine were enrolled. Transdermal clonidine was substituted for oral clonidine and titrated until adequate blood pressure control (seated diastolic blood pressure less than 90 mm Hg) was attained. At the end of titration, seated morning blood pressure averaged 129/90 +/- 15/5 mm Hg (mean +/- standard deviation) compared to 136/96 +/- 13/7 mm Hg (p less than 0.01/0.001) during oral clonidine administration. Standing morning blood pressure was also lower during transdermal than oral therapy (131/94 +/- 16/5 vs 136/99 +/- 14/7, p less than 0.05/0.001). Afternoon blood pressures (at peak effect of oral dose) were virtually identical during oral and transdermal therapy in both seated and standing positions. Typical side effects of oral clonidine, including dry mouth, drowsiness, and sexual dysfunction, were reduced during transdermal therapy. There was less morning-to-afternoon variability of blood pressure control and plasma clonidine concentrations during transdermal than during oral therapy. One patient left the study because of drowsiness and two because of skin reactions to the transdermal skin patch. Mild transient local skin irritation occurred frequently. Transdermal clonidine plus a diuretic is an effective treatment for mild to moderate essential hypertension, improves compliance and reduces side effects of therapy.