We previously showed that conjugated linoleic acid (CLA) increases 15-keto-dihydro-prostaglandin F 2α , a marker for cyclooxygenase-mediated lipid peroxidation and thus an indicator of cyclooxygenase-mediated inflammation. The aim of the present study was to investigate the effects of CLA on other indicators of inflammation in human subjects, including C-reactive protein, TNF-α, TNF-α receptors 1 and 2, and vascular cell adhesion molecule-1. In a double-blind, placebo-controlled study, fifty-three human subjects were supplemented with a mixture (4·2g/d) of the isomers cis -9, trans -11 CLA and trans -10, cis -12 CLA or control oil for 3 months. CLA supplementation increased levels of C-reactive protein ( P =0·003) compared with the control group. However, no changes in TNF-α, TNF-α receptors 1 and 2, and vascular cell adhesion molecule-1 were detected.