Results of five years of continuous monitoring of airborne pollen concentrations with a volumetric trap in Leiden indicate that almost 36% of the total pollen count originates from a group of 23 taxa, herbs and weeds flowering in the summer, between May and October. More than 95 % of the airborne summer-weed pollen are fromUrtica, Rumex, Artemisia, Plantago, and Chenopodiaceae,Urticapredominating with over 85%. Of the three most frequent summer-weed pollen types, bothRumexandUrticashow variable seasonal patterns, whereasArtemisiais more constant. Although large concentrations ofUrticapollen are often found, allergic sensitization is infrequent. The converse is true ofArtemisiapollen, which could be a cause of pollinosis in August in spite of the relatively low concentrations in the air.