US researcher Robert Bailey of the University of Illinois at Chicago put forward long-term data from a trial in Kisumu, Kenya, that in its initial phase enrolled 2,784 uncircumcised uninfected men.
Half of the group were circumcised, and the others were circumcised at a later date and they were later tested for HIV.
Previously-published research from this trial found that, after two years, circumcised men were 59-percent less likely to contract the human immunodeficiency virus (HIV) than uncircumcised counterparts.
The benefit was so astonishing that at this 24-month mark, the uncircumcised men were offered circumision, as it would have been unethical not to have done so.
In a presentation at the 17th International AIDS Conference, Bailey said the estimate of protectiveness at 24 months had been adjusted to 60 percent in the light of refined lab tests from blood specimens.
And he also reported that at the 42-month mark, circumcision offered a protectiveness of 65 percent.
This extension of the study, conducted among 1,739 of the original 2,784 volunteers, will run until December 2009.
"The 60-percent protective effect against HIV acquisition ... over the first 24 months of the study, we now find to be sustained and possibly strengthened to approximately 65 percent over three and a half years of follow-up," Bailey said.
"These results further support the addition of male circumcision to our limited armamentarium of HIV prevention."
Meanwhile, South African researcher Dirk Taljaard reported on a new aspect of a now-famous circumcision trial at Orange Farm, South Africa.
This French-led trial was the first to show that surgical removal of the foreskin offered protection against HIV, also around 60 percent.
Taljaard said that analysis of other data from the Orange Farm trial showed that circumcision offered no shield to the man against gonorrhea.
It offered only "borderline" protection against trichomonal vaginitis, a parasite that dwells in the male and female sexual organs and is transmitted through vaginal intercourse.
But it provided protection of 36 percent against the human papillomavirus (HPV), a pathogen linked to cancer of the cervix and penis, he said.
Circumcision has emerged as one of the few bright areas of progress in the AIDS crisis, after the Orange Farm and Kisumu trials.
The discovery has sparked talk in some circles of a "surgical vaccine" -- a cheap, safe method for shielding men from HIV, especially in sub-Saharan Africa, home to two-thirds of the 33 million people with HIV.
But this enthusiasm has also been tempered by worries that circumcision may face a backlash on cultural, religious or sexual grounds, or may prompt men to abandon use of the condom.
Other work at the conference, ending Friday, suggested, though, that circumcision was widely accepted by men, some of whom believed that it enhanced sexual pleasure, and did not prejudice safe-sex practices.
According to French researcher Bertran Auvert, who led the original South African study, circumcision could avert up to 3.8 million infections and half a million deaths in sub-Saharan Africa between 2006 and 2016, and up to 5.8 million deaths by 2026.
The theory behind the effectiveness of circumcision is that the inner foreskin is an easy entry point for HIV. It is rich in so-called Langerhans cells, tissue that the AIDS virus finds particularly easy to latch onto and penetrate.
Other questions surrounding a circumcision campaign are the need to ensure that operations are done hygienically and with the full knowledge and consent of the male. Also unclear is what benefit, or otherwise, male circumcision has on women.