Conjugated linoleic acid (CLA) is a term used for a mixture of fatty acids with conjugated double bonds that could have beneficial properties on human health. From a quantitative point of view, rumenic acid (RA, cis-9 trans- 11 C18:2) is the predominant isomer in foods containing fat derived from ruminants, mainly dairy products. Most of the biological properties of CLA have been attributed to RA, especially the anti-tumoral effects. In order to assess the role of this molecule in carcinogenic processes a large number of research comprising studies in vitro with cell cultures, in vivo with animal models and epidemiological in humans has been undertaken. This article reviews the state of art of such bioactive lipid naturally present in milk and its role in the inhibition of different type of cancers. Although most of the studies reviewed support the idea of RA as functional ingredient in the cancer prevention, more research, mainly in humans, is needed to confirm these evidence.