PURPOSE OF REVIEW: Isomeric fatty acids have the same number of carbon and hydrogen atoms, but may have distinct metabolic and health effects. Two well-known examples of isomeric fatty acids are cis and trans monounsaturated fatty acids, and conjugated isomers of linoleic acid (CLA). The purpose of this review is to summarize recent findings from human studies on the metabolic and health effects of these two classes of isomeric fatty acids. RECENT FINDINGS: Apart from an unfavorable effect on serum lipoproteins, trans monounsaturated fatty acids from hydrogenated oils may increase plasma markers for a low-grade inflammatory state. From epidemiological studies, however, it is not possible to conclude if effects of ruminant and industrial trans fatty acids on cardiovascular risk are different. In contrast to in-vitro and animal studies, there are no indications that in humans the two most common CLA isomers (cis9,trans11-CLA and trans10,cis12-CLA) affect body composition differently. Longer-term supplementation, however, may slightly decrease body fat mass without apparent effects on plasma markers for glucose and lipid metabolism. Other studies have even reported adverse effects of CLA supplementation on insulin resistance and lipid peroxidation. SUMMARY: Evidence is increasing that trans monounsaturated fatty acids from hydrogenated oils increase plasma markers of low-grade chronic inflammation. From epidemiological studies, however, it is not clear if effects of ruminant and industrial trans fatty acids on cardiovascular risk are different. Effects of CLA on body composition remain controversial and more research is needed before the widely available CLA supplements should be advocated as an adjunct to control body weight.