We studied the effects of elevated concentrations of carbon dioxide ([CO₂]) and ozone ([O₃]) on growth, biomass allocation and leaf area of field-grown O₃-tolerant (Clone 4) and O₃-sensitive clones (Clone 80) of European silver birch (Betula pendula Roth) trees during 1999–2001. Seven-year-old trees of Clones 4 and 80 growing outside in open-top chambers were exposed for 3 years to the following treatments: outside control (OC); chamber control (CC); 2 × ambient [CO₂] (EC); 2 × ambient [O₃] (EO); and 2 × ambient [CO₂] + 2 × ambient [O₃] (EC+EO). When the results for the two clones were analyzed together, elevated [CO₂] increased tree growth and biomass, but had no effect on biomass allocation. Total leaf area increased and leaf abscission was delayed in response to elevated [CO₂]. Elevated [O₃] decreased dry mass of roots and branches and mean leaf size and induced earlier leaf abscission in the autumn; otherwise, the effects of elevated [O₃] were small across the clones. However, there were significant interactions between elevated [CO₂] and elevated [O₃]. When results for the clones were analyzed separately, stem diameter, volume growth and total biomass of Clone 80 were increased by elevated [CO₂] and the stimulatory effects of elevated [CO₂] on stem volume growth and total leaf area increased during the 3-year study. Clone 80 was unaffected by elevated [O₃]. In Clone 4, elevated [O₃] decreased root and branch biomass by 38 and 29%, respectively, whereas this clone showed few responses to elevated [CO₂]. Elevated [CO₂] significantly increased total leaf area in Clone 80 only, which may partly explain the smaller growth responses to elevated [CO₂] of Clone 4 compared with Clone 80. Although we observed responses to elevated [O₃], the responses to the EC+EO and EC treatments were similar, indicating that the trees only responded to elevated [O₃] under ambient [CO₂] conditions, perhaps reflecting a greater quantity of carbohydrates available for detoxification and repair in elevated [CO₂].