Revenue from Internet advertising in the U.S. hit a record $31 billion last year, according to a study released Wednesday.

That's up 22 percent from $26 billion in 2010, the previous record.

The Interactive Advertising Bureau, an industry group, conducted the quarterly study with PricewaterhouseCoopers.

About half of the ad revenue, $14.8 billion, came from the search category. Those are the text-based ads that are sold by Google and others and that are targeted to search terms and other keywords.

The fastest-growing category was mobile, with revenue of $1.6 billion in 2011, more than double the $600 million a year earlier. It's still a small part of overall Internet advertising, however.

David Silverman, a partner with PricewaterhouseCoopers, said the growth in mobile should continue given its ability "to deliver timely, targeted, relevant and local advertisements in a manner that was not previously possible."

According to the study, retail advertisers were the largest spenders, accounting for 22 percent, or $7.1 billion, of the total last year.

Internet ad revenue in the final three months of 2011 totaled $9 billion, a 20 percent increase from the same period in 2010. That's also a record, beating the $7.8 billion in the third quarter of 2011.

Explore further US Internet ad revenue hit record in 2010

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