Background: Although controlled laboratory experiments have been conducted to demonstrate the sensitivity of allergenic pollen production to future climatic change (ie, increased CO 2 and temperature), no in situ data are available. Objective: The purpose of this investigation was to assess, under realistic conditions, the impact of climatic change on pollen production of common ragweed, a ubiquitous weed occurring in disturbed sites and the principal source of pollen associated with seasonal allergenic rhinitis. Methods: We used an existing temperature/CO 2 gradient between urban and rural areas to examine the quantitative and qualitative aspects of ragweed growth and pollen production. Results: For 2000 and 2001, average daily (24-hour) values of CO 2 concentration and air temperature within an urban environment were 30% to 31% and 1.8° to 2.0°C (3.4° to 3.6°F) higher than those at a rural site. This result is consistent with most global change scenarios. Ragweed grew faster, flowered earlier, and produced significantly greater above-ground biomass and ragweed pollen at urban locations than at rural locations. Conclusions: Here we show that 2 aspects of future global environmental change, air temperature and atmospheric CO 2 , are already significantly higher in urban relative to rural areas. In general, we show that regional urbanization-induced temperature/CO 2 increases similar to those associated with projected global climatic change might already have public health consequences; we suggest that urbanization, per se, might provide a low-cost alternative to current experimental methods evaluating plant responses to climate change. (J Allergy Clin Immunol 2003;111:290-5.)