The effectiveness of transdermally administered clonidine using Catapres-TTS patches applied once a week was assessed in the control of mild to moderate hypertension by 16 general practitioners in 135 subjects. Following two weeks on placebo patches, subjects with mean seated diastolic pressures 90 to 104 mmHg were titrated to one, two or three patches and blood pressure responses measured at monthly intervals for three months. Satisfactory response of blood pressure to 90 mmHg or lower was obtained in 85% of patients who completed the maintenance phase; in 20 patients whose blood pressure responses did not reach 90 mmHg, a significantly higher blood pressure (mean 158/103 mmHg) occurred at entry into the trial (145/96 mmHg). Withdrawal from the trial was mainly for reasons of inadequate blood pressure control (15%) or localised skin reactions (15%). Dryness of the mouth was the only systemic side effect encountered with significant frequency and resulted in the withdrawal of three patients. Localised skin reactions were encountered in 51% of subjects, in the majority these were mild and subjects elected to continue with transdermal clonidine because of the convenience of once a week administration. Severe or generalised skin reactions were not encountered. Serum biochemical measurements, including serum lipid levels, remained unaltered. Transdermal clonidine therapy provided satisfactory blood pressure control in the majority of subjects and apart from the localised skin reactions which were largely of nuisance value, proved safe and acceptable to patients.