The small study from California's Los Angeles County found that only 61 percent of home kitchens would get an A or B if put through the rigors of a restaurant inspection. At least 14 percent would fail - not even getting a C.
"I would say if they got below a C, I'm not sure I would like them to invite me to dinner," said Dr. Jonathan Fielding, director of the Los Angeles County Department of Public Health.
In comparison, nearly all Los Angeles County restaurants - 98 percent - get A or B scores each year.
The study, released Thursday, is believed to be one of the first to offer a sizable assessment of food safety in private homes. But the researchers admit the way it was done is hardly perfect.
The results are based not on actual inspections, but on an Internet quiz taken by about 13,000 adults.
So it's hard to use it to compare the conditions in home kitchens to those in restaurants, which involve trained inspectors giving objective assessments of dirt, pests, and food storage and handling practices.
What's more, experts don't believe the study is representative of all households, because people who are more interested and conscientious about food safety are more likely to take the quiz.
"You'll miss a big population who don't have home computers or just really don't care" about the cleanliness of their kitchens, said Martin Bucknavage, a food safety specialist with Penn State University's Department of Food Science.
A more comprehensive look would probably find that an even smaller percentage of home kitchens would do well in a restaurant inspection, he suggested.
In 2006, the county health department began a home kitchen self-inspection program, designed to help consumers learn how to store and prepare food safely. The department also began offering an online quiz with 45 yes or no questions that simulates a restaurant inspection checklist.
People are asked, for example, if their refrigerator temperature is 41 degrees Fahrenheit or lower, whether raw meat is stored below other foods on refrigerator shelves, and whether fruits and vegetables are always thoroughly rinsed before they are eaten.
The study is based on quizzes taken through 2008.
Overall, 34 percent got an A, meaning they correctly answered at least 90 percent of the questions. Another 27 percent got a B, 25 percent a C, and 14 percent failed to score at least a 70.
An estimated 87 million cases of food-borne illness occur in the United States each year, including 371,000 hospitalizations and 5,700 deaths, according to an Associated Press calculation that uses a CDC formula and recent population estimates.
Many outbreaks that receive publicity are centered on people who got sick after eating at a restaurant, catered celebration or large social gathering. In this summer's outbreak linked to salmonella in eggs, several illnesses were first identified in clusters among restaurant patrons.
But experts believe the bulk of food poisonings are unreported illnesses from food prepared at home.
The study is being published in Morbidity and Mortality Weekly Report, a publication of the Centers for Disease Control and Prevention.