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 today.

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.