The efficiency of the clonidine transdermal therapeutic system was assessed in a group of patients under 35 years of age and in another group of patients over 60 years of age. The patients in both groups had mild uncomplicated hypertension. Blood pressure (BP) was measured by the three following methods: by the physician; by an automatic device at the outpatient clinic; and by the patient at home. Seventeen young patients (mean age, 26 years) and 18 elderly patients (mean age, 68.5 years) were included in a double-blind crossover study comprising four periods of 2 weeks each, during which patients were given, weekly, either placebo or one and then two transdermal clonidine patches containing 2.5 mg of the drug. In the younger group, the average systolic and diastolic BPs measured by the physician after administration of two clonidine patches were, respectively, 4.9 and 5.9 mm Hg lower than after placebo (p less than 0.05). The absolute drop was more significant in the older group (9.3 and 6.0 mm Hg; p less than 0.01). In the young group, placebo markedly reduced systolic BP (9.8 mm Hg; p less than 0.01), and comparison of the three methods of BP measurement demonstrated their great reactivity to the physician's presence. Nine young and eight elderly patients received clonidine treatment for an average duration of 9 +/- 5 months. Six cases of skin reactions occurred and limited the usefulness of the clonidine transdermal system used in this study.