Ambulatory blood pressure profiles were obtained with the Remler M2000, a portable semiautomatic blood pressure recorder, in 38 chronically treated hypertensive patients who continued to have blood pressures measured by their physician greater than 140 mm Hg systolic and greater than 89 mm Hg diastolic. On the average, ambulatory recorded blood pressures were significantly lower (151/94 +/- 26/13 mm Hg; mean +/- SD) than those determined at the clinic not only by a physician (179/109 +/- 22/11 mm Hg), but by a nurse (163/101 +/- 24/10 mm Hg). Individual mean recorded ambulatory blood pressures could be predicted neither from office readings obtained by a physician nor from those measured by a nurse. Because of this unpredictability of blood pressures prevailing outside the clinic, ambulatory blood pressure monitoring seems to be very useful, if not necessary, in assessing the efficacy of antihypertensive drugs. By this technique, it may be possible to select patients who do not need a change of treatment although their blood pressure levels remain persistently elevated in the physician's office.