Although the earliest races involved relatively small groups of messengers, recent contests have included large numbers of participants who are not messengers and whose skills vary. While some veterans have welcomed the newcomers, others have suggested that the influx has diluted the purity of the races. Then there are those who maintain that less experienced riders may be placing themselves in harm’s way by trying to keep up with seasoned messengers who are accustomed to whizzing through traffic at top speed.

More than 100 racers had been expected to take part in the Monster Track alleycat. Entry was limited to people riding track bikes that are not equipped with hand brakes, and thus require an extra level of expertise. Other events connected to the Monster Track at sites in Brooklyn and Queens, like a bike tricks competition and a track race, were still scheduled.

“We decided to cancel the main part of the race because we were concerned that it was getting dangerous,” said Kevin Bolger, one of the Monster Track organizers. “When we heard about what happened in Chicago we were really stunned.”

But some riders believe that the death in Chicago was more of an aberration than an inevitability, and they insist that alleycats and similar contests will flourish in New York and elsewhere.

Supporters say that the races, replete with what one bicyclist described as “mavericks, outlaws, rapscallions and misfits,” should continue precisely because they are the sort of brash, raffish events that seem to be increasingly rare in a more antiseptic New York. Many also see the races as important rallying events that foster camaraderie among bicyclists and encourage the wider public to use nonpolluting transportation.

Among those who would like to see the contests continue is Michael Green, who maintains the Web site BikeBlog (bikeblog.blogspot.com) and made a short documentary, “Track Kaiju,” about a messenger from Tokyo who, he said, finished fifth in the 2007 Monster Track.

“It’s cultural propaganda that promotes cycling,” Mr. Green said. “The loss of that would be monumental.”