One fifth of the patients found to have borderline high blood pressure in a New York medical clinic returned to normal blood pressure by the time they got home, a study reports.

The finding suggests that "white coat hypertension" -- where the anxiety of a visit to the doctor's office apparently prompts an increase in blood pressure -- is much more common than previously thought.

It also means that many of the estimated 15 percent of adult Americans thought to have very mild hypertension may in fact have normal blood pressure -- with no need for treatment by anti-hypertension drugs that sometimes cause side effects.

In light of the high percentage of "white coat hypertensives," doctors should consider getting an at-home blood pressure reading for borderline patients before putting them on a strict diet or anti-hypertension medication, researchers said.

The study, by the Cardiovascular Center at New York Hospital-Cornell University Medical Center in New York, was reported in the Journal of the American Medical Association.

Researchers studied three groups of patients: volunteers with normal blood pressure, borderline hypertensives and established hypertensives with diastolic blood pressure above 105. None of the patients was under treatment.

For each, blood pressure was not only taken in a clinic (by a male physician and a female technician) but also measured over a 24-hour period by a monitor worn by the patient.

Of the patients who showed borderline hypertension in the clinic, 21 percent -- or more than one in five -- showed a 24-hour blood pressure that was normal.

The "white coat effect," still something of a mystery, is more common in younger patients with a shorter history of hypertension and more common in women than in men, the study found. It is more pronounced when blood pressure is taken by a male doctor than when it is taken by a female nurse or technician.

These differences, the researchers suggest, "might be related to the stereotype of the physician as a male authority figure and the nurse or technician as a female in a more empathic role."

"It has been known for many years that the prognosis of hypertension is better in women than in men," the researchers said. "Our results may also suggest a very simple explanation for this phenomenon: Many of these women are not truly hypertensive."