The Investment Company Institute (ICI) found that assets of US mutual funds hit $10.013 trillion in October this year - a 2.9 per cent increase on September.
The rise was put down by ICI spokesman Edward Giltenan to high levels of investors using mutual funds for long-term investments.
He suggested that more investors were turning to mutuals for the long term than the funds were designed for.
The ICI also revealed stock funds saw assets increase by 3.8 in October to $5.67 trillion.
October also saw long-term funds - including stock, bonds and hybrids - take in $24.37 billion, which was over double the amount for September.
The increases in assets held was put down to the increasing stock market in September and October which continued into November.