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Yarrow, Achillea millefohum L, is one of the commonest weeds of the Compositae family. Cases of allergic contact dermatitis have, been described since 1899. Although 10 sesquiterpene lactones (SL) and 3 polyines have previously been identified, the sensilizers of yarrow have eseaped detection. A reinvetigation of short either extracts of yarrow revealed the presence of 5 unsalurated hitherto unknown guaianolides of peroxide character. Tile main SL. identified as a strong, sensitizer in guinea pig sensitization experiments, was named α-peroxyachifolid. The minor SL also contribute marginally to the sensitizing capacity, while other known yarrow constituents like dehydromatricaria ester and pontica epoxide appear to play no role A 5-year follow-up (1985 1990) of Compositine-sensitive patients showed that more than 50% reacted when tested with a short either extract of ?arrows. Exacerbation of the patch test silos by irradiation with UV light was never observed. [ABSTRACT FROM AUTHOR]