We investigated the content of cis-9, trans-11 isomer of conjugated linoleic acid (CLA) and its seasonal variation in bovine milk fat from Denmark, Finland, Iceland, Norway, and Sweden. Milk samples were collected during winter and summer and analyzed using gas chromatography. The CLA content in milk samples from the Nordic countries ranged from 0.41 to 1.02 g/100 g total fatty acids. The CLA content was higher in Iceland (0.64 +/- 0.02) than in the other countries together (0.57 +/- 0.02), and the concentration in Icelandic and Danish milk (0.63 +/- 0.03) was higher than in Finnish milk (0.48 +/- 0.02) in a comparison of single countries. The CLA concentration during summer was on average 41.6% (11.9) higher than during winter (0.48 g/100 g (0.03) vs. 0.068 g/100 g (0.07)). Season exerted similar changes in all 5 countries. The concentration of CLA in milk fat from the Nordic countries seems lower than in milk from European countries reported previously in the literature, which may be due to shorter summers and longer winters in the Nordic countries. Health implications of varying concentrations of CLA need to be studied.