Three patients with occupational contact dermatitis due toArnica longifoliaEaton andArnica montanaL. are reported. During cultivation, harvesting of the flower heads, chemical investigation of the sesquiterpene lactone constituents and preparation of therapeutic tinctures, they had frequent contact with the plant materials. Patch tests with the two sesquiterpene lactones carabron and helenalin, isolated during investigation fromA. lungifoliaEaton, were positive in all three patients, though the second patient had never shown visible allergic reactions of the skin. Sensitization experiments with carabron in five guinea pigs were successful.The investigation results revealed that carabron, helenalin and the acetyl derivative of helenalin must be considered as the sensitizers ofA. longifolia, and helenalin acetate and properly arnifolin as sensitizeis ofA. motana.Studies on cross reactivity in the sensitized animals with six related sesquiterpene lactones showed that only those compounds gave a positive test response which contain an α‐methylene γ‐lactone group. Cross reactions were obtained with a crude extract ofChrysanthemum indicumL.The case reports supported by the patch test and investigation results demonstrate that persons handling a new drug from the Compositae family run a risk of developing an allergy after intensive contact with the plant and its constit