WASHINGTON — The portion of homes with cell phones but no landlines has grown to 18 percent, led by adults living with unrelated roommates, renters and young people, according to federal figures released Wednesday.

An additional 13 percent of households have landlines but get all or nearly all calls on their cells, the survey showed. Taken together, that means about three in 10 households are essentially reachable only on their wireless phones.

The figures, covering the first half of 2008, underscore how consumers have been steadily abandoning traditional landline phones in favor of cells. The 18 percent in cell-only households compares with 16 percent in the second half of 2007, and just 7 percent in the first half of 2005.

Leading the way are households comprised of unrelated adults, such as roommates or unmarried couples. Sixty-three percent of such households only have cell phones.

About one-third of renters and about the same number of people under age 30 live in homes with only cells. About a quarter of low-income people also have only wireless phones, nearly double the proportion of higher-earning people.

Stephen Blumberg, senior scientist at the federal Centers for Disease Control and Prevention and an author of the report, said there is no evidence the trend is slowing. He said the recession may fuel it further, especially as cell phone prices drop and their coverage and features improve.

"There's clearly a reason to give up a landline phone if budgets are tight," he said in an interview. "Given the current economic environment, I'd not be surprised to see more and more people give up their landline phones for economic reasons."

The findings have major implications for pollsters. In recent months researchers have concluded that people who have only cell phones have slightly different political views than those who do not.

Growing numbers of pollsters now include cell-only users in their samples, which is more expensive in part due to legal restraints against using computers to call them.

The survey also found:

—Just 9 percent of homeowners are cell-only, compared with 34 percent of renters.

—Older people are less likely to have only cell phones, with just 9 percent of those 45-64 and 3 percent of those 65 and up living in such households.

—By race, 22 percent of Hispanic adults, 19 percent of blacks and 15 percent of whites live in cell-only homes.

—The South and Midwest have more cell-only households than the Northeast or West.

People with landlines who seldom take calls on them largely have those phones hooked into computers, or rely so exclusively on their cell phones that they assume anyone calling the landline is a solicitor and seldom answer it.

The data is compiled by the National Health Interview Survey, conducted by the CDC. The latest survey involved in-person interviews with members of 16,070 households conducted from last January through June.