Dr. Klein was admitted to a hospital in Palm Springs 4 days ago ... according to law enforcement sources. We're told he was suffering severe abdominal pains, and remained in the hospital until he died Thursday evening. He was pronounced dead, of natural causes, at 7:50 PM.
Klein, a renowned Beverly Hills dermatologist, had an astonishing fall from grace in the wake of Michael's death. He was the go-to dermatologist for scores of celebrities, including Elizabeth Taylor, Dolly Parton, Goldie Hawn, Dustin Hoffman, Sharon Stone and Cher.
Klein was also a Botox pioneer ... in fact he was called, "The father of Botox."
MJ and Klein were the best of friends, often vacationing and spending holidays together. But MJ used their friendship to help fuel his drug addiction.
Everything unraveled for Klein when Michael died. TMZ broke the story, Klein had injected Jackson with Demerol 51 times in the 3 months prior to his death.
And Klein also took Jackson to various surgery centers to put him under with Propofol, the drug that killed the singer. On one occasion, Klein took Jackson to the gynecologist next door to his office and put him under with anesthesia which one doctor said was Propofol under the pretext it was for an acne treatment. Klein denied it was Propofol.
Klein gave Jackson prescriptions for various narcotic drugs including Demerol under a variety of aliases, including Omar Arnold, Fernand Diaz, Peter Madonie and Josephine Baker.
In the weeks before Jackson's death ... TMZ shot video of Michael at Klein's office almost daily.
Klein was also rumored to be the biological father of Michael's son, Prince Jackson -- and coincidentally, Prince addressed the rumor just yesterday ... essentially saying he knew Michael was not his biological dad, but it didn't matter to him.
As TMZ reported ... Dr. Klein was not Prince's father either, even though Klein once posted a young driver's license photo of himself that stoked the rumors.