Researchers at the University of California, San Francisco reported on Monday, August 25 that HIV-positive men were far more likely than HIV-positive women to transmit the virus to a heterosexual partner. In the study, the male-to-female transmission rate was estimated at 9 out of every 10,000 unprotected sexual encounters and the rate for female-to-male transmissions was lower (see published study for figures). Male-female transmission rates were highest among men with a history of STDs and injection drug use. Of the 77 HIV-positive male injection drug users, 22 percent transmitted the virus to their partners. Of the 163 women who had an STD, 25 percent were infected. The results of the UCSF study were published in the August edition of the American Journal of Epidemiology.