Figures for 2010 released at the Phuket 'Asean City Getting to Zero'summit show more than one in three (34.3 percent) of the HIV cases in Asean countries are in Thailand.
Next was Indonesia, a country with a much larger population, with 20.1 percent, followed by Vietnam with 18.1 percent, Burma with 15.5 percent and cambodia with 4.1 percent.
Singapore (0.2 percent) Laos (0.6 percent) and the Philippines (0.6 percent) barely register.
The Asean plus three conference - adding Japan, China and South Korea - is looking at ways of reducing the burden imposed by HIV.
Latest figures available for 2009 show 530,000 people living with HIV in Thailand, including 210,000 women - by far the highest number in the region.
AIDs-related deaths total 28,000 in Thailand, 18,000 in Burma and 99,000 throughout the region.
The total number of people living with HIV throughout the region was 1,503,600.
The conference is being held at the Movenpick Resort in Phuket's Karon beach district, with Health Ministers from all 13 countries due to convene as the event comes to an end on Friday.