It may not be the sexiest growth category, but in the U.S. and other developed countries, adult incontinence is growing so fast that Procter & Gamble Co. is about to reverse a 15-year-old decision and re-enter the market in a big way, according to industry watchers.

P&G is preparing to launch by this fall a major extension of its Always feminine care brand in adult incontinence after years of testing the move in the U.K. The brand name there was Always Discreet and will likely be the same in the U.S.

P&G will put $150 million in marketing support behind the effort in the U.S. alone, according to Tom Wilson, a former Kimberly-Clark Corp. executive who's now principal in the Caregiver Partnership, where he's blogged about P&G's plans in recent months. Mr. Wilson said he's heard from industry executives that P&G will launch 24 items in pads and underpants, with a three-year commitment to a campaign with heavy concentration on digital, social and mobile media. Leo Burnett handles Always.

Mr. Wilson and Sanford C. Bernstein analyst Ali Dibadj ultimately expect P&G to enter adult incontinence throughout Western Europe, Japan and South Korea as well. It's likely to be a serious challenge to established players that include Kimberly-Clark Corp.'s Depend and Poise, SCA's Tena and Serenity Brands and Kao and Unicharm in Asia among others.

P&G declined to comment, but Mr. Dibadj and other analysts believe adult incontinence was what Mr. Moeller was referring to when he mentioned in a speech at an analyst conference last month that the company would enter a new category by the end of calendar 2014.

Mr. Wilson recalls working on Kotex at K-C when P&G first launched Always in 1984. "We thought we had a great defense plan and they'd never get on the beach at Normandy," he said. "We just got steamrolled."

Mr. Wilson also worked for K-C in 1997 when P&G made a last-ditch effort to build its former Attends adult-incontinence business with a marketing push that, by Mr. Wilson's reckoning, cost more than the brands' annual sales that year. When sales actually declined despite the marketing push, P&G divested the brand the following year, and has been out of the business ever since, aside from occasional tests.

Cached pages for online retailers in the U.K. and online listings for discontinued products show P&G has been testing incontinence products in the U.K. since 2012 under the Always Envive and Always Discreet brand names. It's also sold at least some products in the U.S. earlier this year through Amazon, to those eligible for its PrimePantry program, which launched in April.

Kimberly-Clark Corp. Chairman-CEO Tom Falk highlighted the category's potential in comments on a May earnings call, when he noted that the market has been growing by "high single digits" despite being confined largely to developed markets such as North America and Japan. (In the latter the adult-incontinence market is bigger than the youth diaper market.)

In the U.S., adult incontinence is a $1.4 billion market up 6% in the past year vs. 1% for all of household and personal-care products, according to Nielsen data from Deutsche Bank, which doesn't include Costco or online sales, where the products are also strong.

Mr. Wilson notes that 10,000 boomers turn 65 in the U.S. alone every day, building the potential market even larger. And he believes P&G is smart to go with its billion-dollar Always brand, since 70% of the incontinence market is women. He estimates Always already gets 40% of its maxi pad sales from women using the product for light bladder leakage.

Incontinence pads sell for anywhere from 50% to double the price of feminine pads, Mr. Wilson said, so just getting women who use Always feminine pads to switch to the brand's incontinence products would be a win for P&G -- and retailers.