The present study investigated whether consuming dairy products naturally enriched in cis-9, trans-11 (c9, t11) conjugated linoleic acid (CLA) by modification of cattle feed increases the concentration of this isomer in plasma and cellular lipids in healthy men. The study had a double-blind cross-over design. Subjects aged 34–60 years consumed dairy products available from food retailers for 1 week and then either control (0·17 g c9, t11 CLA/d; 0·31 g trans-vaccenic acid (tVA)/d) or CLA-enriched (1·43 g c9, t11 CLA/d; 4·71 g tVA/d) dairy products for 6 weeks. After 7 weeks washout, this was repeated with the alternate products. c9, t11 CLA concentration in plasma lipids was lower after consuming the control products, which may reflect the two-fold greater c9, t11 CLA content of the commercial products. Consuming the CLA-enriched dairy products increased the c9, t11 CLA concentration in plasma phosphatidylcholine (PC) (38%; P=0·035), triacylglycerol (TAG) (22%; P<0·0001) and cholesteryl esters (205%; P<0·0001), and in peripheral blood mononuclear cells (PBMC) (238%; P<0·0001), while tVA concentration was greater in plasma PC (65%; P=0·035), TAG (98%; P=0·001) and PBMC (84%; P=0·004). Overall, the present study shows that consumption of naturally enriched dairy products in amounts similar to habitual intakes of these foods increased the c9, t11 CLA content of plasma and cellular lipids.