The fast food chain KFC is being sued in the US in a bid to stop it cooking fried chicken in partially hydrogenated oil containing trans fatty acids.
KFC has said it is looking at using other types of oil
The fats have been linked to raised cholesterol and a subsequent increased risk of heart disease.
The case is being brought by the consumer group Center for Science in the Public Interest, which says KFC could use healthier cooking products.
KFC vowed to fight the court case, which it called "frivolous".
The Center for Science in the Public Interest is seeking a variety of economic damages in the case against KFC's parent company, Yum Brands.
The group and Dr Arthur Hoyte, a retired physician from Maryland, want a judge to order KFC to use other types of oils or make sure customers know about trans fat content immediately before they buy food.
Dr Hoyte said he was suing "for my son and others' kids, so they may have a healthier, happier, trans-fat-free future".
He added: "If I had known that KFC uses an unnatural frying oil, and that their food was so high in trans fat, I would have reconsidered my choices," he said.
He and the consumer group are seeking class-action status for their claim and want a judge to let Dr Hoyte represent anyone who ate at a KFC in the Washington DC area in 2004 and 2005.
Michael Jacobsen, director of the centre, said KFC knew it could use healthier fats in its cooking.
But he added: "It recklessly puts its customers at risk of a Kentucky Fried Coronary."
KFC spokeswoman Laurie Schalow branded the lawsuit "frivolous" and said the company would fight it in court.
The company had provided nutrition and fat information to consumers online and in restaurants for a very long time, she said.
And a spokeswoman for KFC GB Ltd added: "KFC provides a wide variety of menu choices and options for our consumers.
"We also provide nutritional information on tray liners in all restaurants and on the KFC UK website."
She added: "We have been reviewing alternative oil options, but there are a number of factors to consider including maintaining KFC's unique taste and flavour, supply availability and transportation.¿