They're increasingly worried about geopolitical instability, as well as the strength of the global economic recovery.
Average cash levels have reached five per cent of portfolios, the highest level since June 2012, and a 4.8 per cent increase from April.
While respondents are optimistic about improvements in both the world economy and corporate performance, many are now questioning the rate of growth.
"Investors are showing belief in the economy but with two big question marks: Are we on the brink of a disruptive event? And why, at this point in the cycle, isn’t this recovery stronger?" said Michael Hartnett, chief investment strategist at BoAML Global Research.
In less than 12 months (April 2015), the current bull market could become the third longest of all-time. But fund managers have identified several potential risks, and this May has seen a shake-up in how respondents assess several of them.
Fear of a China debt default is well up, as are concerns over Eurozone deflation. Far more of those surveyed in May see geopolitical crisis as the biggest potential risk, presumably because of ongoing tensions over Ukraine.