Yarrow, Achillea millefolium 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 sensitizers of yarrow have escaped detection. A reinvestigation of short ether extracts of yarrow revealed the presence of 5 unsaturated hitherto unknown guaianolides of peroxide character. The main SL, identified as a strong sensitizer in guinea pig sensitization experiments, was named alpha-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 Compositae-sensitive patients showed that more than 50% reacted when tested with a short ether extract of yarrow. Exacerbation of the patch test sites by irradiation with UV light was never observed.