The Shamrock Shuffle is the great ‘‘aaah’’ of spring for Chicago runners. And you can look behind the curtain for what has made it the largest 8-kilometer road race in the world.

Registration for the 36th Shamrock Shuffle, which will be run March 29 downtown, starts at 1 p.m. Monday at shamrockshuffle.com. The registration fee is $50. Registration closes when the capacity of 40,000 is reached.

There are several reasons the run has grown from a small local event into the third-largest running event in Chicago and one of the 10 largest in the United States. Foremost among them is a local focus. Ninety percent of the runners come from Chicago and the suburbs.

The setting on an early-spring Sunday helps, too. It’s a perfect time for a running party on the lakefront. And, much like the Chicago Marathon in October, the run draws top runners, though more of them are from the United States than from Kenya and Ethiopia, which dominate the marathon.

Noted U.S. runner Dathan Ritzenhein was favored to win the Shamrock Shuffle in 2013 until an injury forced his withdrawal. Lisa Uhl, a rising figure among U.S. marathoners, won the women’s side that year.

Local favorite Tera Moody, who has won two Shamrock titles, was the women’s favorite in 2014 before an injury forced her to withdraw days before the run. She is expected to compete this year.

In Moody’s absence, Katie Kellner won the women’s side in 2014. Jake Riley, who finished second in 2013, won the men’s race to make it a clean sweep at the top for the Hanson-Brooks Distance Project A in Michigan.