In the mid-1960s, when William R. Fitzsimmons was a student at Harvard, the college took in a freshman class of roughly 1,550, including students at Radcliffe, which it would eventually absorb. In the four decades since, the population of the United States has ballooned by two-thirds, applications to Harvard have tripled and Mr. Fitzsimmons has ascended to the job of dean of admissions and financial aid at Harvard, but this year's freshman class is only about 125 students larger than when he was a student.

That reluctance to grow has been true of many selective colleges that want to sustain their genteel scale. But with ever more students pressing at their gates, admissions officers find themselves having to reject what Anthony W. Marx, Amherst's president, calls "astonishing applicants."

The most elite institutions are accepting historic lows of 10 percent of applicants, and next year the sieve should become excruciatingly finer with applications from baby boomers' offspring expected to crest.

At least four of the nation's most exclusive institutions--Princeton, Yale, Stanford and Amherst--are either modestly expanding enrollments for the first time since the late 1960s (when some began admitting women) or have task forces studying the matter.

For example, Princeton started gradually increasing its freshman classes in 2005, aiming to increase its undergraduate population by 500 students to an enrollment of 5,200 by the fall of 2012. And Stanford, with 6,759 undergraduates, not many more than the 6,571 it had 20 years ago, has appointed a task force to study expansion.

Meanwhile, gauntlets have effectively been thrown down to rival elite colleges by the presidents of Stanford and Yale in recent alumni-magazine articles. Stanford's John Hennessy lamented that its undergraduate population had remained nearly level for 35 years and endorsed a modest expansion as a "practical and principled response to current realities."