The S&P index of US shares was the best performing market in August, while the IA North America Smaller Companies sector was the best performing sector, according to data from FE Analytics.
The US market rallied in recent months after a string of companies reported strong earnings growth, with the S&P 500 returning 3.3 per cent.
Meanwhile the IA North American Smaller Companies sector returned 6.3 per cent, with the continuing bull market attributed to the recent tax cuts made by US president Donald Trump.
The best performing fund was the £88m Miton US Smaller Companies fund, a mandate that only launched in March and returned 12.8 per cent in August. It is run by Hugh Grieves and Nick Ford, who also run the much larger Miton US Opportunities fund.
Meanwhile the £393m Artemis US Smaller Companies fund, run by Irishman Cormac Weldon, was the next best performing fund, returning 10.6 per cent.
Ben Yearsley, director at Shore Financial Planning, said as the US was leading the way in equity markets the strength of the US economy and the dollar was causing problems elsewhere, with Emerging Markets and commodities suffering.
He said: "Annualised growth figures show the US growing at 4.2 per cent - that’s emerging market rates of growth. Add in the first trillion dollar company, Apple, with Amazon closing in on that gargantuan level too and it’s no wonder the US keeps on hitting new highs.
"President Trump likes claiming credit for the stockmarket level and to give him his dues, his tax cuts have been a key driver this year."
A strong performance from the dollar tends to be bad news not only for emerging market assets but for gold as well, and this was borne out by the weakness of funds exposed to those assets in August.
The worst performing fund was Ruffer Gold, which lost 12.4 per cent. As a firm, Ruffer have long believed that the policy of quantitative easing will lead to very high inflation, and that gold will perform well when that happens.
Gold tends to perform badly when the dollar is strong because the dollar competes with gold to act as a safe haven for investors.
Emerging markets meanwhile tend to perform poorly when the dollar is strong because many emerging market economies and countries tend to borrow in dollars, so a stronger performance from the US currency means their borrowing costs rise.
The IA Global Emerging Market equity sector lost 3.15 per cent in August, while the equivalent bond sector lost 3.57 per cent.
The FTSE 100 lost 2.08 per cent, while the FTSE All Share lost 1.73 per cent.
The Euro Stoxx Index of European shares lost 2.21 per cent, partly because European companies tend to have significant exposure to emerging markets, and to move in line with emerging market indices.
Mr Yearsley said: "As ever it pays to be diversified, but if all you had done was own an S&P tracker this year you would have been handsomely rewarded. It is a case of the haves and have nots.