Conjugated linoleic acid (CLA) is a generic term referring to a mixture of geometrical and positional isomers of linoleic acid in which up to 16 members have been identified. Many potentially beneficial health effects have been ascribed to these fatty acids when consumed as a mixture, and where generally 2 isomers dominate, e.g. the 9c, 11t-isomer, the so-called rumenic acid, and the 10t, 12c-isomer: anti-carcinogenic, immune modulator, anti-atherosclerotic, and anti-obesity among the most spectacular. The question arises as to whether the pleiotropic biological activity is supported by one or several of the isomers. Recent studies using pure individual isomers have started to elucidate this issue, but many others are required to ascribe a respective role to each CLA isomer (the main ones as well as the minor ones), such as those occurring in some complex mixtures already commercially available, or even in foodstuff. The aim of the present study was to focus on the CLA-isomer specific effects depicted in the literature up to now.