The Kingdom of Tonga has the greatest proportion of Church members of any country in the world, with about 40 percent of its population being LDS. Other countries with a large proportion of members include Western Samoa with 25 percent, American Samoa with 22.6 percent, and Niue with 15 percent. In the United States, about 1.8 percent of the population is LDS.
Source: Deseret News 1999-2000 Church Almanac, pp. 170, 410.