Microsoft is taking a $6.2bn (£4bn) charge on the value of an online advertising firm bought five years ago.
Microsoft bought aQuantive for $6.3bn cash in an attempt to catch rival Google in the race to grow revenues from search-related advertising.
The write-down effectively wipes out the acquisition's value, although there was little impact on Microsoft's share in after-hours trading on Monday.
The purchase of aQuantive in 2007 was then Microsoft's biggest acquisition.
The company said in a statement on Monday that "the acquisition did not accelerate growth to the degree anticipated, contributing to the write-down".
The charge is likely to wipe out any profit for the company's fourth quarter. Wall Street was expecting Microsoft to report fourth-quarter net profit of about $5.25bn on 19 July.
In addition to the charge, Microsoft said its forecast for future growth and profitability at its online services arm - which includes the Bing search engine and MSN internet portal - are "lower than previous estimates."
http://www.bbc.co.uk/news/business-18684395