More than a quarter of young adults have only cell phones, making them the leading edge of a strengthening move away from traditional landline telephones, a federal survey showed Monday.

Overall, the portion of adults with only cell phones grew by more than 2 percentage points in the latter half of last year to nearly 12 percent, an expansion rate that began in the first part of 2006 and was double earlier rates of growth.

One in four people aged 18 to 24 had only cell phones, as did 29 percent of those aged 25 to 29, the study showed. The percentages declined with age after that, with 2 percent of those 65 or over having only cell phones.

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The trend away from landline phones affects the telephone industry, 911 emergency service providers, and government and private polling organizations, which rely heavily on random calls to households with wired telephones.

"All those wireless adults are missed" in those marketing and opinion surveys, said Stephen Blumberg, senior scientist at the Centers for Disease Control and Prevention and an author of the report.

That's a potential problem, because people with only cell phones tend to be disproportionately young and have lower incomes.

Studies have so far concluded that cell-phone-only users are not a large or diverse enough group to affect the accuracy of broad polls that omit them.

The data, from the CDC's National Health Interview Survey, also found:

— 15 percent of Hispanic adults, 13 percent of black adults, 12 percent of Asians and 11 percent of whites had only cell phones;

— 22 percent of the poorest adults had only cell phones, double the rate for those who are not poor;

— 13 percent of males and 11 percent of females had cell phones only;

— Nearly 2 percent of adults had no phone at all.

The figures were based on interviews with people in 13,056 households from June through December last year.