A Lazard spokesman said: "His condition is serious, but he is stable and recovering."
But shares in the bank have slipped in Frankfurt following the news of Mr Wasserstein's condition.
They fell $0.28 to $41.58 by 07.23am GMT this morning (October 12th 2009).
He arrived at the Lazard Group as a director in January 2002 before becoming chairman and chief executive officer in May 2005.
Under his stewardship Lazard has increased its market share during the financial crisis and posted higher-than-expected profits for the second quarter of 2009.
The bank has recently become the sixth largest advisor in the world on global mergers and acquisitions.
Before joining Lazard, 61-year-old Mr Wasserstein had been a co-founder of advisory firm Wasserstein, Perella & Co in 1988.
The company was sold to Dresden Bank AG for $1.56 billion at the end of 2000.
By Claire Archer