Conjugated linoleic acid (CLA) is a naturally occurring anticarcinogen found in dairy products which is produced in the rumen as an intermediate in the biohydrogenation of polyunsaturated fatty acids by the bacteria Butyrivibrio fibrivsolvens. An experiment was conducted to examine the changes in CLA concentration in bovine milk throughout the year. 338 milk samples were taken from the University of Nottingham's commercial dairy herd between December 1997 and March 1999. All animals followed the same dietary regime throughout the study. Samples were analysed for fat and CLA content. Morning and evening samples were pooled to calculate daily fat and CLA concentrations and yields. There was a small variation in milk yield (P<0.05) whereas CLA content and yield varied considerably (P<0.001) throughout the year (CLA = 0.91 to 2.81 mg/g of fat). The highest concentrations were found during the summer months when the cows were mainly eating fresh grass as opposed to conserved forages. Fat content and yield followed by the month were the predominant factors affecting CLA output. Results indicate that, if required, CLA in milk could be increased through suitable dietary management practices.