Treated or untreated hypertensive patients may have very different blood pressure levels when measured by the doctor in his surgery and when measured by ambulatory monitoring during their usual everyday activities. It is now possible to record blood pressure non-invasively with a portable blood pressure measuring device, the Remler M2000. Using this device, we observed that less than half of the patients referred by their medical practitioner for hypertension had abnormally high blood pressures during their everyday activities. We also noted that the ambulatory blood pressure profile of a given hypertensive patient cannot be predicted from the blood pressure recorded by his or her doctor. Consequently, we concluded that ambulatory blood pressure recordings would appear to be essential for the detection of patients with high blood pressure readings at the doctor's surgery and during everyday activities. These patients probably include the only group which really needs appropriate antihypertensive therapy.