The group, whose rapid expansion of recent years has made it one of the London market's best performing stocks, also warned that margins were likely to be impacted this year by investment costs, including in China.
Its shares dived by 20% at one stage, even though the group said it remains on track to achieve annual sales of more than £1 billion for the first time.
Retail sales were 26% higher year-on-year at £136.7 million in the two months to February 28, but this was below City expectations for 32% growth.
ASOS said sales were strong in all territories except the Rest of World, where it faces adverse currency movements, notably in Australia and Russia.
However, Cantor Fitzgerald analyst Freddie George noted that UK sales growth of 21% to £48.4 million was below his hopes for a rise of 30.6%.