According to reports, Mr Lay was taken by ambulance from his holiday home in Old Snowmass, Colorado, on Wednesday morning and driven to Aspen Valley Hospital.
He and former chief executive Jeffrey Skilling were found guilty in May in connection with Enron's collapse.
Mr Lay was found guilty on 25 May on all six fraud and conspiracy charges that he faced.
He had posted a $5m bond to avoid custody until October when he would have been sentenced - to up to 45 years behind bars.
Mr Lay had always proclaimed his innocence, saying the company was brought down by a lack of investor confidence triggered by revelations that Enron's chief financial officer Andrew Fastow had stolen millions from the company.
