Circumcising men reduced infections in their female partners by 30%, the researchers found. One said the difference may be related to the structure of the foreskin, which is removed in circumcision.
In the study of more than 300 Ugandan couples in which the man infected the woman, the researchers found that 299 women caught HIV from uncircumcised partners and only 44 were infected by circumcised men.
Circumcision also reduced the risk of infection with other sexually-transmitted diseases such as trichomonas and bacterial vaginosis, but not syphilis, gonorrhea or chlamydia, the researchers told an Aids meeting in Denver.
Dr Thomas Quinn of Johns Hopkins University in Baltimore has been leading a team that studies 12,000 volunteers in Rakai, Uganda. They have been studying transmission of the human immunodeficiency virus that causes Aids.
Last year, they reported that circumcised men were less likely to become infected with HIV. Now, they told the Conference on Retroviruses and Opportunistic Infections, it appears that among infected men, circumcision reduces the likelihood they will transmit the virus through sex.
They also presented more evidence that circumcision protects men. They re-analysed previous studies and found that circumcision reduced the risk of HIV infection in men by 50% - and by 70% in the highest-risk men.
The findings will have to be confirmed in other groups before being used as the basis for recommendations, Quinn said.
However, he added, "early indications are dramatic." If borne out, for every 15 to 60 circumcisions, one case of HIV infection could be prevented, he said.