The extent of the problem may go far beyond the number of runners who have been hospitalized for it. A recent study of runners in the 2002 Boston Marathon found that 13 percent who finished the race had hyponatremia. And those were runners who thought they were fine and were just participating in a study. If such a runner continued to drink after the marathon, perhaps thinking that feelings of nausea and malaise were due to dehydration, the runner could end up with seizures or slip into a coma, doctors say.

That is what happened to Mark Robinson, a 27-year-old computer programmer from West Roxbury, Mass., who sees his story as a cautionary tale.

The day of the 2004 Boston Marathon dawned unusually hot. The race was on April 19, but the temperature was projected to reach nearly 90 degrees. Robinson was concerned. It was his first marathon, he had been training for six months, and he wanted to run it in four hours or less. "I sweat a lot," he said. With weather like that, he worried he might become dehydrated. So he tried to make sure he drank enough.

"I drank more than a gallon of water before the race, and then at every rest stop I would stop and have a couple of drinks of water," he said. He was on pace until Mile 19 when, suddenly, he felt nauseous and his legs began to cramp. He forced himself to continue, but by Mile 23 he could no longer run. "I tried to power-walk it in," Robinson said.

His parents met him at the finish line, bringing water. He drank two quarts, but he felt worse than ever. Not only was he vomiting and having diarrhea, Robinson said, but "I felt spacey, out of it, almost like I was on drugs."

His parents got a wheelchair and took him to the medical tent, where the person doing triage at the entrance asked if he could stand on his own. He could. He said he was told, "We have people here who are lying down," and was sent away. His parents helped him walk to the subway and took him to their home in Wayland, Mass. All the while, Robinson was drinking water and drinking Gatorade and vomiting.

Robinson said: "I felt completely mentally out of it. It was a strange sensation. Deep down, I knew something really, really wasn't right. It was like a feeling of impending doom. My father wanted me to take a bath, but I didn't want to be alone. I looked at my dad and he was talking and his mouth was kind of going," but, Robinson said, he could no longer hear what his father was saying.