HE had not expected to spend his 60th birthday in a hospital cardiac unit. R. J. Turner, a commercial real estate broker from Frederick County, Va., had planned a robust celebration. “I was going to finish my 10th marathon,” Mr. Turner said, “which isn’t bad for a guy my age.”

But near the start of the Marine Corps Marathon on Oct. 29, Mr. Turner raised an arm to wave at bystanders, and “everything went black.” Collapsing violently, he gashed his head, chipped a tooth and bit a deep hole in his bottom lip.

Mr. Turner, who had passed a stress test a year before, had just had a heart attack.

This has been an unusual season for the cardiac health of marathoners. After years in which almost no deaths were attributed to heart attacks at this country’s major marathons, at least six runners have died in 2006.

Two police officers, one 53, the other 60, died of heart attacks at the Los Angeles Marathon in March. The hearts of three runners in their early 40s gave out during marathons in Chicago in October, San Francisco in July and the Twin Cities in October. And at the same marathon where Mr. Turner was felled, another man, 56, crumpled near the 17th mile, never to recover.