Despite its reputation for wide-open spaces, the American West is second only to the Northeast in the percentage of residents living in crowded cities and suburbs, and it has acquired a greater public-service burden than any other region, according to a new study.

The report by Westrends, an offshoot of the private, nonprofit Council of State Governments, said the 13 western states have the highest percentage of children and preschoolers in the nation and an elderly population growing three times faster than the national rate.The resulting high "dependency ratio," the report said, could strain public services and force difficult choices about government spending.

The analysis of recent changes in the West noted that before World War II, the region was predominantly rural but now is sharply divided between states with a percentage higher than the national average of people in metropolitan areas and states that fall below that average.

Those states with large urban populations are California, Hawaii, Washington, Colorado, Utah and Nevada. Those states that fall below the national average are Arizona, Montana, Idaho, Wyoming, New Mexico, Oregon and Alaska.

California "is 96 percent metropolitanized," the report said. Figures for 1986 show that the region's 83.7 percent of residents in metropolitan areas was exceeded only by the Northeast's 88 percent.

Westrends president Daniel M. Sprague said the report was designed to challenge several myths about the West so policy-makers and voters could act on more realistic assumptions.

Growth of city and suburban life indicates "a dramatic change in regional character," the report said. "Understanding the region as an urbanized society is at odds with the image many `insiders' and most `outsiders' have of the West."

Such change is the natural result of a population growth rate that, in some ways, contradicts national trends, the report said. Since 1980, the West has had the nation's highest birth rate, with Utah and Alaska at more than twice the national average. From 1980 to 1984, the West was the only region to record an increase in the 5-to-17 age group.

Immigrants and Americans moving to the West from other parts of the country remain an important factor. Fewer than half of current residents were born in the West. Westerners move more often than other Americans but mostly within the region. "For example," the report said, "most of the people coming into Arizona are from California, not from other regions."

The report's San Francisco-based authors, Katherine M. Albert, William B. Hull and Sprague, emphasized the size of the West's "dependency ratio" - the ratio of the population over 65 or under 18 to the working-age population.

The national ratio is .62 and is exceeded by Utah (.83), Idaho (.76), New Mexico (.68), Montana (.67), Arizona (.66), Wyoming (.66) and Oregon (.66).

The report, "The Dynamic West: A Region in Transition," also analyzed economic changes. Sprague emphasized suggestions for cushioning dizzying economic shifts, such as turning failed ranches or mining communities into resorts to tap the region's mushrooming tourist market.