We have treated 128 patients aged 40 +/- 9 years (60 males and 68 females), all with essential hypertension (W.H.O. I and II), over a period of 10 yr. The treatment was performed with clonidine at a dose that ranged from 0.150 to 1,200 mg (twice daily). Forty-two patients also received a diuretic (HCTZ 25 mg daily). Mean blood pressure decreased significantly from 169 +/- 10 mm Hg systolic, 107 +/- 3 diastolic to 145 +/- 6 mm Hg (p less than 0.001) 90 +/- 3 mm Hg diastolic (p less than 0.001). Side effects occurred during the first month. These were drowsiness 28%, dry mouth 35%, constipation 13%, dizziness 9%, postural hypotension 2%, and male impotence 3.3% (2/60). Side effects still present after 120 months of treatment were drowsiness 11.7%, dry mouth 26.6%, constipation 14.1%, dizziness 4.7%, and male impotence 1.7% (1/59). The number of patients who discontinued treatment resulting from side effects were 3.34%, all of them within the first 6 months. There were no changes in renal or liver function or in serum electrolytes or lipids. Retinopathy improved in most patients. Electrocardiogram (ECG) improved in 45 patients with LVH. It is concluded that clonidine provided sustained blood pressure control with minimum side effects during 10-year therapy for hypertension.