AbstractThis article critically reviews the existing, although limited, literature concerning trans fatty acids and tumorigenesis. Neither epidemiological nor experimental studies published to date have demonstrated any valid association between trans fatty acid ingestion and tumorigenesis. A recent study showed that under controlled conditions, a fat with a high content of‘trans fatty acids did not promote the development of mammary tumors induced in rats by 7,12‐dimethylbenz[a]anthracene to any greater extent than did a comparable fat with a high content of as fatty acids. In addition, in this study a high trans fat was less tumor promoting than was a blend of fats that simulated the dietary fat composition of the United States and had a lower level of trans fatty acids. Another study using comparable cis and trans fats demonstrated that the high trans fat did not affect the growth and metastasis of implanted mammary tumors in mice relative to the high cis fat. Also, two recent studies reported no significant difference in the development of induced colon tumors in ratsfed diets high in cis or trans fatty acids. The results of these and other studies are consistent with the conclusion that trans fatty acids are not uniquely related to tumor development.