The US stockmarket watchdog SEC will investigate computer services firm IBM over one of its earnings reports and its stock options scheme.
The move gives the SEC the right to see internal IBM e-mails, documents and to interview company executives.
IBM said it was co-operating with the investigation, but in after-hours trading its share price fell 1.3%.
The investigation focuses on how IBM accounted for stock options in a quarterly earnings report a year ago.
At the end of its first quarter in 2005, the company had fallen 5 cents short of the average Wall Street estimate – an unusual situation for IBM which usually is better at managing investor expectations.
However, analysts soon realised that the real shortfall was much higher – about 9 cents below the expected profit per share – but only once they understood how IBM had accounted for stock options for company workers.
Last week IBM made headlines when it announced the closure of its final-salary pensions scheme to existing members, giving them personal pensions instead.
The company said the change, from January 2008, would give it more control of its pension costs as well as saving around $500,000m a year.
IBM has moved beyond its roots as a computer and software firm, with consulting services now an important driver of company profits.