This paper describes observations on the airborne pollen concentrations of four anemophilous weeds of very common occurrence in Western Europe, viz. Rumex spp. (sorrel and dock), Plantago spp. (plantain), Chenopodiaceae (goosefoot family) and Artemisia spp. (mugwort). Comparative results are recorded from London, Leiden, Brussels. Munich and Marseilles for the summers of 1976 and 1977 and this study is one of a series involving international collaboration between workers in cities of the European Economic Community. The literature on the clinical significance of summer weed pollen is reviewed and it is concluded that the pollen of Artemisia is likely to be of importance in areas where the weed is of common occurrence in late summer. The pollen of the other summer weeds is unlikely to be important, but may contribute to the problems of the pollen sensitive patient under exceptional circumstances.