Parthenium allergy is caused by direct and indirect contact with Parthenium hysterophorus L. of the Compositae family, accidentally introduced into India from the USA 20 years ago. Its distribution was supported by the optimal environmental conditions of this country. It causes severe contact dermatitis in a lot of people, which becomes chronic after longer exposition. The source of the epidemic are the strong sensitizing sesquiterpene lactones parthenin, ambrosin and others, which are highly concentrated in the trichomes of the plant. During the dry season in India the mature plants crumble to a fine dust, that is scattered by the wind and becomes disseminated throughout the countryside. Permanent contact with this dust is the source of this socalled "airborne contact dermatitis". About 12 deaths occurred in severely affected persons due to intercurrent infection. Control of Parthenium hysterophorus is possible with Ansar-529, a weed-killer containing arsenic. Recently the weed has been introduced into Central-Australia too.