['The potential effects of global climate change on allergenic pollen production are still poorly understood.', 'To study the direct impact of rising atmospheric CO2 concentrations on ragweed (Ambrosia artemisiifolia L.) pollen production and growth.', 'In environmentally controlled greenhouses, stands of ragweed plants were grown from seed through flowering stages at both ambient and twice-ambient CO2 levels (350 vs 700 microL L(-1)). Outcome measures included stand-level total pollen production and end-of-season measures of plant mass, height, and seed production.', 'A doubling of the atmospheric CO2 concentration stimulated ragweed-pollen production by 61% (P = 0.005).'] Is production of allergenic pollen by ragweed ( Ambrosia artemisiifolia L. ) increased in CO2-enriched atmospheres?