ASOS said it would have to step up spending on technology and logistics to help maintain its lead in the online fashion pack, with the extra costs taking a toll on its elevated share price.
The UK company increased its capital expenditure forecast after reporting a 27% rise in sales to £1.13 billion in the six months to the end of February.
ASOS said capex for the full year was now expected to be £230-250m, higher than the £200-220m it predicted in January. Investment will remain at the higher level in its next financial year.
The company, which is still listed on London's junior AIM market, has an eyewatering price-earnings ratio of 92.
Its market capitalisation of £5.9 billion is more than £1.5 billion higher than that of retail stalwart Marks and Spencer.
ASOS chief executive Nick Beighton said the company was trading strongly, with visits to its site exceeding 1 billion in the six months.
"Alongside our investment in our people and our technology, we are accelerating investment in our distribution and logistics, laying the foundation for £4 billion of net sales," he said.
Beighton said the extra money was needed to keep upgrading its 200 localised websites, and incorporate more artificial intelligence into services like its recommendations engine and visual search.
It will also open its warehouse in Atlanta in the US in July, three or four months earlier than planned, he said.
ASOS, like its online fashion rivals Zalando and Boohoo.com, has focused on rapid sales growth through investment, prizing market share above all.
The company today reported pretax profits of £29.9m, just shy of average market forecasts of £30.8m.
It kept its forecast for around 25-30% reported sales growth for the year unchanged, along with its target of an operating profit margin of about 4%.