Field-grown yellow-poplar (Liriodendron tulipifera L.) werefumigated from May to October in 1992–96 within open-topchambers to determine the impact of ozone (O3) aloneor combined with elevated carbon dioxide (CO2) on saplinggrowth. Treatments were replicated three times and included: charcoal-filteredair (CF); 1 × ambient ozone (1 × O3);1·5 × ambient ozone (1·5 × O3);1·5 × ambient ozone plus 350 p.p.m.carbon dioxide (1·5 × O3 + CO2)(target of 700 p.p.m. CO2); and open-air chamberlessplot (OA). After five seasons, the total cumulative O3 exposure (SUM00 = sumof hourly O3 concentrations during the study) rangedfrom 145 (CF) to 861 (1·5 × O3) p.p.m. × h (partsper million hour). Ozone had no statistically significant effecton yellow-poplar growth or biomass, even though total root biomasswas reduced by 13% in the 1·5 × O3-exposedsaplings relative to CF controls. Although exposure to 1·5 × O3 + CO2 hada stimulatory effect on yearly basal area growth increment aftertwo seasons, significant increases in shoot and root biomass (∼ 60% increaserelative to all others) were not detected until the fifth season.After five seasons, the yearly basal area growth increment of saplingsexposed to 1·5 × O3 + CO2-air increasedby 41% relative to all others. Based on this multi-yearstudy, it appears that chronic O3 effects on yellow-poplargrowth are limited and slow to manifest, and are consistent withprevious studies that show yellow-poplar growth is not highly responsiveto O3 exposure. In addition, these results show thatenriched CO2 may ameliorate the negative effects of elevatedO3 on yellow-poplar shoot growth and root biomass underfield conditions.