This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
According to a section of "True Blue," the writers quickly ran
out of room for the character of Laura Kelly, especially once
they realized the chemistry between Dennis Franz and Sharon
Lawrence and began writing more scenes for Sylvia Costas.
Sherry was a good sport about it, and went to Bochco and Milch
with the suggestion that letting her out of her contract might
be the best for everyone. They did so, and Sherry got a job as
Dr. Susan Lewis on NBC's mega-hit "ER."
Sherry left "Blue" because she didn't have enough to do; two
and a half years later, she would leave "ER" because she had
too much to do, and wanted to settle down and have a normal