The guitarist was 64 when he died March 6. He was diagnosed with prostate cancer in 2007.
E! News confirmed the cause of his death Wednesday.
"By now, the devastating truth of Ronnie's death is public knowledge," said a statement posted by Montrose's family on his Web site. "We hope you can understand why we wanted to keep this news a private family matter for as long as possible. We can only hope that you will choose to celebrate Ronnie's life, and what his music meant to you, rather than his passing. Ronnie would have wanted it that way."