They work in hotels, saunas, massage parlours and karaoke bars; an army of 300,000 whose existence is unacknowledged, whose activities are illegal, and whose industry is irresistibly lucrative and impossible to wipe out.
Dongguan has become known as the sex capital of China, where shrewd operators - abetted by accommodating officials - have built a prostitution empire that is estimated to contribute 20 to 30 per cent of the service industry's total output.
Under orders from the Ministry of Public Security, municipal authorities this month embarked on a high-profile crackdown to rid the city of this stain, which the party chief admitted was 'disgraceful'. But clamping down is not as simple as closing brothels, jailing operators and removing women.
When an estimated one in 10 migrant workers - between 500,000 and 800,000 people - are involved in the sex industry, the impact of a major crackdown would be intense, especially in a manufacturing hub that was buffeted harder than almost any other region by the economic downturn. Added to this is the resistive power of vested interests: wealthy operators, corrupt officials and others who profit from the industry.
Prostitution was outlawed by the Communist Party after it took power in 1949, but since the opening up began three decades ago, the world's oldest profession made a comeback. Restrictions were relaxed by local authorities, who could either profit from it or who lacked sufficient resources to keep it at bay.
In Dongguan, the scale of the industry attracted Beijing's attention. The city was given until the end of this month to get its house in order; otherwise it would be given the worst crime ranking by inspectors from the Central Committee for Comprehensive Management of Public Security.