Because of its ability to induce contact dermatitis, the establishment and spread of poison ivy is recognized as a significant public health concern. In the current study, we quantified potential changes in the biomass and urushiol content of poison ivy as a function of incremental changes in global atmospheric carbon dioxide concentration (CO2). We also examined the rate of new leaf development following leaf removal to simulate responses to herbivory as functions of both CO2 and plant size. The experimental CO2 values (300, 400, 500. and 600 mu mol mot(-1)) corresponded approximately to the concentration that existed during the middle of the 20th century, the current concentration and near and long-term projections for this century (2050 and 2090), respectively. Over 250 d, increasing CO2 resulted in significant increases in leaf area, leaf and stem weight, and rhizome length relative to the 300 mu mol mot(-1) baseline with the greatest relative increase occurring from 300 to 400 mu mol mot(-1). There was a nonsignificant (P Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0. 18) increase in urushiol concentration in response to CO2; however, because of the stimulatory effect Of CO2 on leaf biomass, the amount of urushiol produced per plant increased significantly for all CO2 above the 300 mu mol mot(-1) baseline. Significant increases in the rate of leaf development following leaf removal were also observed with increasing CO2. Overall, these data confirm earlier, fieldbased reports on the CO2 sensitivity of poison ivy but emphasize its ability to respond to even small (similar to 100 mu mol mot(-1)) changes in CO2 above the mid-20th century carbon dioxide baseline and suggest that its rate of spread, its ability to recover from herbivory, and its production of urushiol, may be enhanced in a future, higher CO2 environment. Nomenclature: Poison ivy, Toxicodendron radicans (L.) Kuntze TOXRA.