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POPULATION TODAY. 1992 Feb; 20(2):6-7.The 1990 world population extensions, published by the United Nations Population Division, differ dramatically from the 1980 long-range projections and suggest reason for serious concern about future trends. In the 1980 extensions, it was assumed that all countries would achieve replacement fertility. The 3 scenarios developed--high, middle, and low--resulted in a population estimate for the year 2150 that ranged from 10,139 to 28,025 billion. In contrast, the 1990 extensions show 7 divergent long-range scenarios that yield totals for 2150 ranging from 4,299 to 694,213 billion. The latter projection assumes no change in current population growth rates. the medium-high series, based on the assumption that fertility will settle on a value 5% above replacement level, yields an estimate of 20,772 billion for 2150; the medium-low series, which projects a fertility level 5% below replacement level, produces a population estimate of 5,633 billion. In both the 1980 and 1990 series, the medium estimate was based on achievement of replacement-level fertility on a schedule that varied region by region on the basis of current birth rates, population policies, and socioeconomic factors. This estimate for 2050 increased from 10,139 billion in 1980 to 11,543 billion in the 1990 series, due to upward revisions in life expectancy in the latter calculations. These projections are of great significance since they form the basis for food production, labor force, and other socioeconomic planning.