This article is from the ER FAQ, by Rose Cooper email@example.com with numerous contributions by others.
In the episode "Sleepless in Chicago" , Dr. Morganstern
announces that he`s going to be leaving Cook County to head up a new
residency program in Boston. His replacement, Dr. William Swift,
appears two episodes later ("Full Moon, Saturday Night"), and it
looked like Morgenstern was gone for good.
Swift was given the name "Wild Willie" because Tag, who knew
him at Ohio State, mentions that they used to call him that. Indeed,
this nickname grew to be so pervasive that many "ER" viewers seem to
only remember him by it.
However, in the first episode of the second season ("Welcome
Back, Carter" ), suddenly Swift was gone, and Morgenstern was
back. The reason for this is that in between seasons, Michael
Ironside, who played Dr. Swift, was offered the lead role on NBC`s
science fiction series "SeaQuest 2032" (replacing that show`s former
lead, Roy Scheider). He took the job, and helmed the sinking series
during its third and final season.
No explanation was offered on the show as to where Swift
went, although we did get a reason why Morgenstern returned :
the residency program he was supposed to lead was fouled up by
bureaucracy, and never got off the ground.
...but, proving once again that in TeeVee Land, a character
shouldn't be counted out until they are dead (and even then...), Wild
Willie returned unto the fold: in "Think Warm Thoughts" ,
Swift returned to the ER as an Attending Physician for Synergix, this
time without much fanfare. And, just as quickly, left.