The patients were remarkably similar: They were all between the ages of 8 and 10, all complained of headaches that had lasted two to three days, and all had pain that fluctuated throughout the day.

One of the patients also had neck and wrist pain. None, however, had a fever, nor did they have anything that might indicate an underlying infection or neurological problem.

As far as outbreaks go, this little cluster hardly matched SARS or even the chicken pox. But Dr. Howard J. Bennett, a pediatrician and clinical professor of pediatrics at George Washington University Medical Center in Washington, D.C, was intrigued.

Bennett soon ascertained the patients did have one other thing in common. All three were reading Harry Potter and the Order of the Phoenix, the fifth installment in the enormously popular series by J.K. Rowling.

His diagnosis? Hogwarts Headache, or, as Bennett writes in a letter to the editor in the Oct. 30 issue of the New England Journal of Medicine, “a tension headache brought on by the effort required to plow through an 870-page book.” That’s a three-pound, 870-page book.

The latest Harry Potter saga exceeds the first by a good 500 pages and more than a pound. “Each of Rowling’s successive books has been bigger than the last,” Bennett writes. “If this escalation continues as Rowling concludes the saga, there may be an epidemic of Hogwarts Headaches in the years to come.”

Bennett’s letter is accompanied by a neatly drawn graph illustrating the burgeoning size of each Harry Potter book. That graph could perhaps just as easily be demonstrating the increased incidence of Hogwarts Headaches as Harry’s school tale unfolds.

Hogwarts Headaches join a long list of other “hazards of daily living,” many of which have found their way over the years into the letters column of the venerable journal. They include Nintendo Tendonitis, Water Skiers’ Enema (when you fall a certain way water enters your rectum), Hunan Hand (inflammation from touching capsaicin-containing chili peppers), Penile Frostbite (winter-time damage to the tip of a jogger’s penis) and Jogger’s Nipple (from the shirt rubbing up and down).

The series of Hogwarts Headaches, Bennett noticed, started about four to six weeks after the fifth volume came out. Two of the patients lay on their stomachs to read the book. One lay on her back with the book propped up against her knees and her head on a pillow. One of the children had put in six to eight hours a day to finish the book.

Bennett’s prescription (to take a break from reading now and then) was resoundingly rejected by two of the patients. They opted for a short course of acetaminophen. The pain went away one to two days after the book was finished.

“In no way should this deter children from reading Harry Potter or encyclopedias for that matter,” Bennett stresses. “Reading is good.”

Some medical advice, however, may be in order. If you get a headache from reading, Bennett says, take a break. It’s always better to read at a chair or a desk than on your stomach with your head cocked. And good lighting will reduce eye strain.

Or wait for the paperback.

Scholastic, Harry Potter’s publisher, did not comment on the findings.