Even though his two seizures occurred 14 years apart, they meet the criteria for epilepsy because they were “unprovoked,” meaning that they were not caused by a head injury, a drug reaction or another known factor.

About 2.7 million people in the United States have epilepsy, and in 70 percent of the cases the cause is unknown, according to the Epilepsy Foundation. Neurologists sometimes describe seizures as an electrical storm in the brain, a brief episode of heightened activity that can cause mild symptoms that are barely noticeable, or loss of consciousness and convulsions, as in the case of the chief justice.

“If he takes medication, he is likely to do fine and be seizure-free,” said Dr. Gregory L. Barkley, a neurologist who specializes in epilepsy and is clinical vice chairman of the neurology department at Henry Ford Hospital in Detroit.

Dr. Barkley said he did not expect the illness or the treatment to affect the chief justice’s ability to do his job.

But if Chief Justice Roberts’s two seizures were so far apart, is another really likely, or likely to occur soon enough to warrant taking a drug every day? Neurologists differ.

“I would recommend taking medication,” Dr. Barkley said. “The intervals tend to get shorter and shorter, and people tend to have recurrent seizures.”

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He explained: “The brain learns from practice. The more you practice, the better you get, whether you’re playing the piano or having seizures. The more you have, the more you’re going to have. Most neurologists feel that the best way to intervene is to get the seizures under control as quickly as possible.”

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Dr. Robert S. Fisher, director of the epilepsy center at Stanford University and a past president of the American Epilepsy Society, said: “In my view, it would be reasonable not to treat. It sounds like he went 14 years between seizures, and that’s a lot of pills to take to prevent the next seizure 14 years from now. The new ones are better than the old ones in terms of side effects, but they all have potential side effects and risks.”

Doctors say a complete medical workup is needed to find out if the two reported seizures were really the only ones that have occurred, because people with epilepsy can have mild seizures that they are not aware of. Neurologists often ask family members whether patients have certain symptoms, like daydreaming or blanking out for brief intervals, and not snapping out of it when others try to speak to them. Other symptoms may include dizziness, sensations in the stomach, feelings of déjà vu or noticing odd smells or tastes in the mouth — experiences that the patient may not recognize as seizures.

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Often, Dr. Barkley said, a patient will report having had one seizure, but when asked about these other symptoms will say, “Oh, yes, that happens all the time.”

If such symptoms are frequent, it may sway the decision in favor of treatment.

Dr. Cynthia L. Harden, a professor of neurology at NewYork-Presbyterian Hospital/Weill Cornell Medical Center, said it would be important to perform brain-wave studies to look for abnormal electrical activity in the brain, to get an idea of where the seizures may be starting and how frequent they are.

Dr. Harden said such studies during sleep were especially useful, and could help in making the decision about treatment. In people with epilepsy, Dr. Harden said, sleep deprivation can be a potent cause of seizures. The chief justice has had a busy schedule recently, including a two-week trip to Europe during which he attended conferences and taught.

Epilepsy is a common condition among Americans. The risk of having a single seizure in one’s lifetime is 9 percent. By Chief Justice Roberts’s age, 52, the risk is 7 percent to 8 percent. About 3 percent of patients have a recurrence, and 1 percent of Americans at any one time are under treatment for epilepsy.

Judd Jensen, a neurologist at Penobscot Bay Medical Center who officials said treated Chief Justice Roberts, declined to answer any questions late yesterday afternoon in his office, which he shares with two neurologists and two cardiologists. “I can’t answer those questions,” Dr. Jensen said.

Dr. W. Allen Hauser, a professor of neurology and epidemiology at Columbia University, estimated that the chief justice had about a 60 percent chance of having another seizure at some point, based on research on other patients who had had two seizures.

“I would say there is no standard recommendation,” Dr. Hauser said. “With multiple seizures, my recommendation generally is to put patients on antiseizure medication, and I think that would be the recommendation of most neurologists.

“That said,” he went on, “the reason one doesn’t immediately put everybody on the medications when they have seizures is that there are side effects to all the medications.”

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“Fifteen percent will have some problem with the first drug that will lead to discontinuation and a different drug,” Dr. Hauser added. “Virtually everybody will have some side effects.”