Conjugated linoleic acid (CLA) has been shown in experimental studies to have chemoprotective properties, and may decrease the deposition of body fat. CLA is prone to oxidation, and it has been suggested that increased lipid oxidation may contribute to the anti-tumorigenic effects of this agent. The present study investigates the urinary levels of 8-iso-prostaglandin F(2alpha) (8-iso-PGF(2alpha)), a major isoprostane, and of 15-oxo-dihydro-PGF(2alpha), a major metabolite of PGF(2alpha), as indicators of non-enzymic and enzymic arachidonic acid oxidation respectively after dietary supplementation with CLA in middle-aged men (mean age 53 years) with abdominal obesity for 1 month in a randomized controlled trial. Significant increases in the levels of both 8-iso-PGF(2alpha) and 15-oxo-dihydro-PGF(2alpha) in urine (P<0. 0001 and P=0.0013 respectively) were observed after 1 month of daily CLA intake (4.2 g/day) as compared with the control group. The lipid peroxidation parameters had returned to their basal levels at 2 weeks after the cessation of CLA intake, and remained at the same levels for a further 2 weeks until the end of the study. CLA had no effect on serum alpha-tocopherol and gamma-tocopherol levels, or on the urinary levels of 2,3-dinor-thromboxane B(2). Thus CLA may induce both non-enzymic and enzymic lipid peroxidation in vivo in middle-aged men with abdominal obesity, without any side effects. The consequences of the increased lipid peroxidation after CLA supplementation are unknown.