Compositae dermatitis in an allergic contact dermatitis, frequently chronic, triggered by plants of the Compositae family. The condition, which is seen, in particular, in middle-aged and elderly persons, typically starts in the summer months with an acute eczema either corresponding to areas exposed to light and air, or corresponding to areas of plant contact; it disappears spontaneously during the fall of the year. Repeated exposure over a number of years may cause severe intractable dermatitis affecting the patient throughout the year. The responsible allergens are sesquiterpene lactones, and investigations to date indicate that the frequency of the allergy is similar to that seen with some of the contact allergens of the standard patch test series. The diagnosis, however, is difficult, partly due to the fact that the clinical picture may be mistaken for a photodermatosis, and partly owing to the difficulty in obtaining commercially available allergens for patch testing. The diagnosis is therefore based on testing with plant extracts, which can cause pronounced local reactions and possibly active sensitization. A survey is given of the possible modes of sensitization, sex and age distribution, the relationship to photosensitivity and atopy, as well as the possible treatment. It is concluded, on the basis of the chronic nature of the disease, and the diagnostic and treatment problems, that continued intensive research into the condition is urgently needed.