Some believe this could remove a big support for share prices. Alastair Winter at Daniel Stewart & Co says the Fed's injections of cash "have boosted equity prices". Hans Olson at Barclays points to low volatility in share and bond prices and warns that "cheap and ubiquitous money breeds complacency" - which could be punctured by the withdrawal of QE.
But not everyone agrees that QE has pushed prices dangerously high. Fed chair Janet Yellen recently told Congress that most equity valuations are "generally in line with historical norms". Michael Hanson at Bank of America Merrill Lynch points out that although price-earnings ratios are slightly above their long-term average, they are far below the levels reached in the 2000 bubble.
What's more, although monetary support for shares is being withdrawn, another support might be emerging - stronger economic activity. Figures next week are expected to show that the US economy expanded well in the second quarter, and that employment and manufacturing activity are growing healthily. Yi Wen at the St Louis Federal Reserve says that because the ending of QE has been widely expected, it might already have depressed economic activity. If so, it won't do so much in future.
If the economy does grow well, equities could cope with withdrawal of QE. Some, though, doubt this will happen. "It is the worldwide slowing of growth that most worries me," says Mr Winter. "I just cannot foresee the scale of profits that are built into most equity markets around the globe."
