Twenty-one percent of 292 patients with untreated borderline hypertension (clinic diastolic blood pressures persistently between 90 and 104 mm Hg) were found to have normal daytime ambulatory pressures (defined from a population of normotensive subjects). These patients were defined as having "white coat" hypertension, and they were more likely to be female and younger, to weigh less, and to be more recently diagnosed than patients whose pressure was elevated both in the clinic and during ambulatory monitoring. Patients with white coat hypertension did not show a generalized increase of blood pressure lability, nor an exaggerated pressor response while at work. The phenomenon is more pronounced when blood pressure is measured by a physician than by a technician. In such patients, the pressor response may be relatively specific to the physician's office and lead to significant misclassification of hypertension.