In some major cities in the US, the underground sex economy accounts for as much as $300 million a year.
According to a significant study, funded by the National Institute of Justice and carried by Washington-based Urban Institute, erotic massage parlours and brothels in eight major US cities raked in between $39.9 million and $290 million from 2003 and 2007.
"With knowing the size of the economy, you get a better sense of what you are dealing with and how big this market is," Meredith Dank, a researcher at the Washington-based Urban Institute, was quoted as saying.
In major cities such as Miami, the underground sex economy was estimated as $302 million in 2003.
In Atlanta, it was $290 million in 2007.
The underground sex economy in Washington, DC, accounted for $100 million in 2007.
Law enforcement now knows they can potentially seize such amount of money that can be used toward providing services and education, the study noted.
The eight cities were Kansas City, Seattle, Dallas, Miami, Atlanta, Denver, San Diego and Washington, DC.
Researchers conducted extensive field research and interviews with former pimps, sex traffickers, sex workers and law enforcement officials.
"We are not trying to make a statement that these cities need to worry because they are a hub for this," Dank said.
"Part of it is regional diversity in saying this is happening everywhere, there is a lot of money being made no matter what city you go in," she added in a National Journal report.
