An autopsy report found The Lost Boys star, 38, had flu-like symptoms before he passed away.
'The pneumonia is what killed him,' says coroner's spokesman Craig Harvey.
An enlarged heart, damaged lungs and narrowed blood vessels also contributed to the ruling of death by natural causes.
While drugs and prescription medication were found in Corey's system, the coroner says it played no part in his passing.
'Nothing was at a level that would have contributed to his death,' Mr Harvey tells the Associated Press.
Corey was declared dead at an LA hospital after he collapsed at his mum's home on 10 March.
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