The predominant CLA isomer found in milk fat is cis-9, trans-11, and it represents 80 to 90% of the total CLA found in dairy products. This isomer is an intermediate in ruminal biohydrogenation of linoleic acid, and a portion escapes the rumen and is incorporated in milk fat. In addition, cis-9, trans-11 CLA is synthesized from trans-11 C18:1, another intermediate in ruminal biohydrogenation. This reaction involves the enzyme (-9-desaturase, and accounts for over two-thirds of the CLA in milk fat. Many dietary factors are known to affect the CLA content of milk fat. Among these factors, the dietary addition of plant oils (sunflower, soybean, canola, and linseed) results in substantial increases in milk fat concentration of cis-9, trans-11 CLA. In general, plant oils high in linoleic acid give the greatest response. An increase in milk fat concentration of CLA is also observed with dietary addition of fish oils, fishmeal or marine algae. The technology is therefore available to produce high-CLA milk under commercial conditions. This production would require the dietary addition of vegetable over the entire lactation. The influence of long-term feeding of high fat diet on cow health and performance remains to be determined.