Aversion to milk products may have no real basis but exists only in the mind. Such people may be labouring under stress, anxiety or depression.
The discovery has important bearing on health, because many who believe they are lactose intolerant leave out dairy products from their diet.
This could leave them severely short of calcium, raising the odds of brittle bones and falls and fracture in old age.
The study is the latest to question whether many of the millions of people who claim to have food intolerance are actually fussy eaters, the Daily Mail reports.
Previous research concluded that nine in 10 Britons who believed they had a food allergy or intolerance were actually perfectly healthy.
Twenty percent of men and women, some 10 million British adults, claim to be unable to eat foods from milk to mustard, but fewer than two percent actually have a problem.
Guido Basilico, of the University of Milan, tested more than 100 people who had stomach ache, bloating and diarrhoea and believed they were lactose intolerant, to see if they really did have problems breaking down the sugar and absorbing it into their blood.
He also asked them about their mental and physical health, including whether they were depressed or anxious or suffered from general aches and pains.
This revealed their stomach troubles to have little to do with digestion of lactose and more to do with their mental state.
These findings were presented at the Digestive Disease Week conference in the US Sunday.