Fast-casual restaurant concepts are among the fastest-growing U.S. companies, says this year's Inc. 500, a list of the fastest-growing, privately-held U.S. companies according to percentage revenue growth from 2008 to 2011.

A few quick-service brands such as Rita's Italian Ice and Bojangles were represented on the list, but as with 2011, most of the restaurants represented were part of the fast-casual segment. Smashburger, based in Denver, was 99th overall last year, but is at number 405 this year, with a three-year growth of 928%. The better-burger chain's 2011 revenue was $53.1 million, compared to $5.2 million in 2008. It was founded in 2007.

Numerous restaurant brands and franchise groups that made the top 500 list include:

At number 1094, Utah-based Heirloom Restaurant Group has grown by 290% with $2.4 million in revenues in 2011. The company manages the Communal, Pizzeria Seven Twelve and Mountain West burrito restaurant groups.

San Francisco-based Paxti's Pizza, with 271% growth in three years, ranked number 1,161. The restaurant specializes in Chicago-style, deep-dish pizza and has several locations in the Bay Area.

Genghis Grill Franchise Concepts, at number 1,290, has grown by 240%. The Dallas-based company's 2011 revenues were $100.1 million, compared to $29.5 million in 2008. The restaurant specializes in "build-your-own" Asian stir-fry and exhibition-style cooking.

Dallas' Dickey's Barbecue Restaurants' revenues jumped from $66.2 million in 2008 to $175.2 million in 2011, a 164% increase. The company, ranked at number 1,734, offers slow-smoked meats and home-style side dishes.

Texas' Wingstop experienced a 50% jump and 2011 revenues of $44 million, good enough to finish at number 3,605 on Inc.'s list.

To qualify for the list, companies must have been founded and generating revenue by March 31, 2008. They had to be U.S.-based, privately-held, for profit and independent—not subsidiaries or divisions of other companies—as of Dec. 31, 2011. The minimum revenue required for 2008 is $100,000; the minimum for 2011 is $2 million. Companies on the Inc. 500 are featured in Inc.'s September issue and online.