Actually there were five battles in the Civil War with more casualties:
#1
Battle of Gettysburg
Date: July 1-3, 1863
Location: Pennsylvania
Confederate Commander: Robert E. Lee
Union Commander: George G. Meade
Confederate Forces Engaged: 75,000
Union Forces Engaged: 82,289
Winner: Union
Casualties: 51,112 (23,049 Union and 28,063 Confederate)
#2
Battle of Chickamauga
Date: September 19-20, 1863
Location: Georgia
Confederate Commander: Braxton Bragg
Union Commander: William Rosecrans
Confederate Forces Engaged: 66,326
Union Forces Engaged: 58,222
Winner: Confederacy
Casualties: 34,624 (16,170 Union and 18,454 Confederate)
#3
Battle of Chancellorsville
Date: May 1-4, 1863
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: Joseph Hooker
Confederate Forces Engaged: 60,892
Union Forces Engaged: 133,868
Winner: Confederacy
Casualties: 30,099 (17,278 Union and 12,821 Confederate)
#4
Battle of Spotsylvania
Date: May 8-19, 1864
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: Ulysses S. Grant
Confederate Forces Engaged: 50,000
Union Forces Engaged: 83,000
Winner: Confederacy
Casualties: 27,399 (18,399 Union and 9)000 Confederate)
#5
Battle of Antietam
Date: September 17, 1862
Location: Maryland
Confederate Commander: Robert E. Lee
Union Commander: George B. McClellan
Confederate Forces Engaged: 51,844
Union Forces Engaged: 75,316
Winner: Union
Casualties: 26,134 (12,410 Union and 13,724 Confederate)
Source is:
http://www.civilwarhome.com/Battles.htm