Monohydroxy, dihydroxy-, and trihydroxyoctadecenoic acids in beer and wort were simultaneously determined using gas chromatography after a solid extraction method. These three acids were detected at ppm levels in the wort. The monohydroxyoctadecenoic acids were not detected after wort boiling, but the dihydroxy- and trihydroxyoctadecenoic acids were transferred through wort boiling, fermentation and lagering into the finished beer. During the mashing using a laboratory mash bath, they gradually increased to about twice the levels those at mashing-in. The amounts of dihydroxyoctadecenoic acid and trihydroxyoctadecenoic acid in commercial beer samples varied from 0.6 to 1.6 ppm and 6 to 15 ppm, respectively.