The United States was home to the most millionaires in 2009 -- 2.87 million -- followed by Japan with 1.65 million, Germany with 861,000, and China with 477,000.
Switzerland had the highest concentration of millionaires: nearly 35 for every 1,000 adults.
Yet as portfolios bounced back, investors remained wary after a collapse that erased a decade of stock gains, fueled a contraction in the global economy and sent unemployment soaring.
The report, based on surveys with more than 1,100 wealthy investors with 23 firms, found that the rich were well served by holding a broad range of investments, including commodities and real estate.
"The wealthy allocated, as opposed to concentrated, their investments," Merrill Lynch head of U.S. wealth management Lyle LaMothe said in an interview.