Data released by the research group said 18 percent of mortgage holders without equity loans had a home where more is owed on the loan than the house's current value, The Wall Street Journal reported Tuesday.
Among homeowners who took out home equity loans, lines of credit or other subordinate loans, however, 38 percent have homes considered underwater, CoreLogic said.
A variety of other data help define the problem. Home prices are down 34 percent from their peak value in 2006, an S&P/Case-Shiller home price index shows.
At the same time, the National Association of Realtors estimates it will take 9.2 months to sell all the homes listed on the market, provided homes sell at the current rate of sales.
That indicates the market has stalled.
Across the country, the number and percentage of homes underwater dropped from the fourth quarter of 2010 to the first quarter this year. But there were still 10.9 million homes in that situation January through March, CoreLogic said.
"When a homeowner's house is underwater, it's harder to get a credit card or a car loan. You can't put your home up for a small business loan. There are all sorts of little, pernicious effects that you don't necessarily think about," Moody's Analytics chief economist Mark Zandi said.