By Ania Lichtarowicz
BBC Health correspondent
Men who have been circumcised may be six times less likely to contract the HIV virus than uncircumcised men, research carried out in India suggests.
A global campaign is under way to increase awareness about Aids
The study in the Lancet journal says that the thin foreskin tissue could be highly prone to HIV infection.
The latest study, which backs up earlier research in Africa, was carried out among 2,000-plus men in India.
Researchers say circumcision only reduces the risk of HIV infection - other sexual diseases are not affected.
A number of studies have shown that circumcision appears to lower the chances of contracting HIV.
When Aids first began to emerge in Africa, researchers found that it was more prevalent in the east and south of the continent than in the west.
Differences in sexual behaviour were widely thought to be reason for this.
But some scientists argued that as circumcision was more common in west Africa, it could be reducing the risk of HIV infection, as the foreskin could be more susceptible to the virus than other parts of penis.
This latest research, looking at more than 2,000 men in India, suggests exactly that.
Circumcision only seems to have a protective effect against HIV - other sexually transmitted diseases are just as likely to be passed on, say the researchers.
They believe this may be because the foreskin contains cells that the virus specifically targets.