The Oscar-winner, who was last seen on the long-running medical drama in 2000, denied last year he would make an appearance before the series' close this spring. But sources told People on Wednesday the star was set for a guest spot, and that ER co-creator John Wells has ordered a closed set.
Neither an NBC rep nor a studio rep for the show would confirm Clooney's return for TVGuide.com.
Last week, NBC announced that ER's season would be extended by three episodes, a move seen by many as a means to woo the sexy Dr. Doug Ross back to County General. The series concludes April 2.
Clooney's return would be among a list of favorites to guest-star in the last season. Anthony Edwards appeared early this season as Dr. Mark Greene in cleverly reconstructed flashbacks revolving around Angela Bassett's Dr. Cate Banfield.
How would you like to see Clooney scrub back in?