An open study was carried out in general practice on 678 hypertensive patients being treated with a beta-blocker but who still had resting diastolic blood pressures greater than 100 mmHg. Previous treatment was stopped and substituted with 1 or 2 tablets daily of a fixed combination product containing 160 mg oxprenolol hydrochloride in a sustained-release formulation plus 0.25 mg cyclopenthiazide per tablet. After 4-weeks' treatment on this regimen, blood pressure levels fell significantly, from 179/108 mmHg to 155/93 mmHg, without any increase in the reporting of unpleasant side-effects.