The annual Los Angeles Marathon routinely draws more than 20,000 participants every year, many of them athletes from around the world.
The 26.2-mile race can be grueling for those in the best physical shape, but especially people who suffer from heart conditions or other physical ailments.
On Sunday, four out of 86 people treated, mostly runners, suffered potentially life-threatening medical problems that required immediate care, said L.A. Fire Department Capt. Erik Scott.
That doesn't count individuals who visited medical tents along the marathon course, which stretched from downtown Los Angeles to the Santa Monica Pier.
There were no deaths, but the conditions of the four patients were not immediately known.
In 2006, two runners died in the Los Angeles Marathon.
Raul Reyna, 53, a veteran LAPD detective, collapsed two miles from the finish line. It was his fifth marathon.
The other marathoner was Jim Leone, a 60-year-old retired Los Angeles County sheriff's deputy from St. George, Utah, who died of an apparent cardiac arrest after collapsing in the third mile.
Since those deaths, race officials have taken steps to try to prevent such tragedies by making defibrillators available and setting up more medical tents along the way.
The marathon has had one other known death: William McKinney, a 59-year-old runner from Altadena, who suffered a heart attack in the 1990 race.