A spokeswoman said he was admitted to hospital in Fort Lauderdale on Wednesday after suffering pain.
He is reportedly suffering kidney failure and needs a liver transplant.
Earlier this year the singer said he had dementia and would stop touring in order to "enjoy life".
A spokeswoman for Cassidy told the Press Association that he was conscious, following reports he had been put into a coma.
She said he was surrounded by family.
The singer's health has been deteriorating for more than two months, entertainment website TMZ reported.
