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The study, published in this week's Journal of the American Medical Assn., found that among children whose blood pressure readings indicated hypertension, only 1 in 4 had the diagnosis documented in their medical records.

If the findings are extrapolated nationwide, as many as 1.5 million children and teens could have undiagnosed high blood pressure, said Dr. David C. Kaelber, a pediatrician at Boston Children's Hospital and a study coauthor. That compares with about 500,000 children who have been correctly diagnosed.

"I expected to find some under-diagnosis, but it was the magnitude of the under-diagnosis that was most striking," Kaelber said. He said the study was "a wake-up call for providers as well as parents that we need to become much more educated and careful about looking at blood pressure in children."

Other research already has suggested that high blood pressure is becoming more common in children and adolescents in the U.S., paralleling the nation's obesity problem. The estimated prevalence is between 2% and 5%, making it a relatively common chronic disease for children.

Blood pressure refers to the force exerted on artery walls as blood flows through the body. Untreated hypertension usually does not cause life-threatening problems in children because the damaging effects occur over time, but it has been linked to heart attacks, strokes and kidney failure in adults.

Normal blood pressure for adults is a systolic pressure below 120 and a diastolic pressure of 80 as measured in millimeters of mercury. Systolic pressure between 120 and 139 or a diastolic pressure between 80 and 89 means pre-hypertension, or slightly elevated pressure that often leads to hypertension.

The study examined the records of more than 14,000 patients, ages 3 to 18, who were seen at least three times between June 1991 and September 2006 in the Cleveland area.