Revised figures in the latest study cut the estimate to 32.7 million from 39.5 million cases.
It says most of the decrease is due to a revision of the statistics from India.
The number of people worldwide infected with HIV in 2007 totalled 2.5 million people and 33.2 million are now living with the virus, the report said. More than two million people died from the disease in 2007.
The report said the number of people living with the virus was levelling out and the percentage of the population affected was now in decline.
Two-thirds of new infections were recorded in sub-Saharan Africa, although the overall number now infected in the region, where three-quarters of the world's AIDS deaths have occurred, was down by 1.7 million this year.
About 22.5 million people living in Africa have HIV/AIDS, 68% of the global total.
In Asia there are now 4.9 million cases, up 440,000 from last year.
Indonesia has the fastest growing epidemic on the continent, while the number of HIV cases in Vietnam has more than doubled between 2000 and 2005.
The Caribbean is the second worst-hit region of the world in per capita terms with 1% of adults, 230,000 people, carrying the virus.
