Intake of conjugated linoleic acid (CLA) has been demonstrated to beneficially affect risk markers of atherosclerosis and diabetes in rats. CLA is naturally found in milk fat, especially from cows fed a diet high in oleic acid, and increased CLA intake can occur concomitantly with increased milk fat intake. Our objective was to investigate the effect of CLA as part of a diet rich in butter as a source of milk fat on risk markers of atherosclerosis, inflammation, diabetes type II, and lipid peroxidation. A total of 38 healthy young men were given a diet with 115 g/d of CLA-rich fat (5.5 g/d CLA oil, a mixture of 39.4% cis9, trans11 and 38.5% trans10, cis12) or of control fat with a low content of CLA in a 5-wk double-blind, randomized, parallel intervention study. We collected blood and urine before and after the intervention. The fatty acid composition of plasma triacylglycerol, cholesterol esters, and phospholipids reflected that of the intervention diets. The CLA diet resulted in increased lipid peroxidation measured as an 83% higher 8-iso-prostaglandin F2alpha concentration compared with the control, P < 0.0001. We observed no other significant differences in the effect of the interventions diets. In conclusion, when given as part of a diet rich in butter, a mixture of CLA isomers increased lipid peroxidation but did not affect risk markers of cardiovascular disease, inflammation, or fasting insulin and glucose concentrations.