Revenues from mobile "adult services" are set to approach $3.5bn by 2010, according to a new report.
Juniper Research said that growth will be fuelled by increasing adoption of streamed video and video chat, and a sharp rise in the adoption of 3G services.
A significant proportion of new revenues are expected to come from the relatively underdeveloped North American markets, despite restrictions on on-portal content.
"While operators in the US and Canada are still very reluctant to introduce age-verification systems and offer adult content, it is a completely different story off-portal," said report author Dr Windsor Holden.