More than 8,000 delegates from 18 countries are expected in the Italian city for the seventh International Conference on AIDS.
The six-day session is considered the most important forum for the exchange of information on the disease, which has claimed hundreds of thousands of lives since first identified a decade ago.
The Florence meeting is expected to be less raucous than last year's international conference in San Francisco, which often resembled a political convention as AIDS activists protested the U.S. government's response to the epidemic.
In Florence, organizers expect the focus to move back to science. Although incremental progress toward better treatments and an AIDS vaccine may be announced, however, the worldwide AIDS epidemic remains largely out of control.