Most articles mention trans fats as having a bad effect on cholesterol and cardiovascular health. But because they are very inflammatory, they are also implicated in cancer. They are believed to increase risk of colon and breast cancers, as well as diabetes and strokes.
An article on breastcancer.org said, "A study found that eating a lot of trans fats may increase breast cancer risk. Of the 25,000 European women who participated in the study, women who had the highest levels of trans fats in their blood were about twice as likely as women with the lowest trans fat levels to develop breast cancer."
Another large study of 18,000 people showed an increased risk of death from all causes in people who ate high amounts of trans fats.