Chicago follows a first-come, first served registration policy.  Registration closes when the race reaches its capacity in terms of the number of runners. Since 2007, the race capacity has been 45,000 runners. In recent years, as running has become more popular, the race has filled earlier (this despite that fact the number of runners was recently increased).

Registration typically opens on February 1 each year.  The following chart gives the date the race closed in recent years:

In 2007, 2008 and 2009, the Chicago Marathon closed within 2-5 days after the running of the Boston Marathon. In 2010 the race closed even earlier, a mere 1 month and 22 days after registration opened on 2/1.  The 2011 edition of the race set a new record, closing in 1 month, 2 days.  Most recently, 2012 set another record, with registration closing a mere 5 days after it opened on February 1.