This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
"ER" ended its fourth season one place behind "Seinfeld", and is
still currently the highest-rated one-hour show airing in the United States.
[And since "Seinfeld" is no longer with us...].
The highest-rated episodes of "ER" to date were "Hell and High
Water" , airing the week of 11/13/95, and the season four finale,
"A Hole in the Heart" , shown originally on 5/14/98. "Hell and High
Water" captured a 28.4 rating and a 47 share, which means that 47% of all
people watching television in the U.S. were watching "ER"; "A Hole in the
Heart" garnered a Nielsen 28.8/45 rating/share ratio. With that, the
finale just barely squeaked by "Ambush" , aka "ERLive", the live
fourth season premier which had a Nielsen rating/share of 28.5/45 with an
audience of 42.7 million U.S. viewers.
Current-week Nielsen ratings can be found on the "USA Today" web pages
<http://www.usatoday.com/life/enter/tv/letindex.htm>. The rec.arts.tv FAQ
<http://members.aol.com/IJBall/WWW/ratv/FAQL.html> explains, in some detail,
why full Nielsen ratings are not available on the 'net.