Clonal birch (Betula pendula Roth.) seedlings were exposed to slightly higher than ambient levels of ozone and different levels of nitrogen supply under open-field conditions in three experiments: (1) three 2-year-old clones exposed to ozone and 74/150 kg N ha−1 y−1 for two growing seasons, (2) three 2-year-old clones under a corresponding exposure regime for one growing season and (3) one 2-year-old clone exposed to ozone and 37/74/150 kg N ha−1 y−1 for one growing season. The cumulative ozone exposures were 1–5 x (1992) and 1–7 x (1993) higher than ambient levels. In these experiments no significant differences between the clones were found in their response to each treatment. Ozone exposure reduced the mean leaf size and the leaf area, accelerated autumn yellowing and loss of leaves, and increased the palisade layer thickness. High nitrogen availability increased the height growth, the mean leaf size, the leaf area, the shoot/root ratio, the leaf water content and the stem dry weight, but decreased the number of leaves and the leaf dry mass per area, and retarded autumn senescence of leaves. It also reduced the leaf and palisade layer thickness and increased the intercellular space of the palisade layer. Significant interactions between ozone and nitrogen supply were found as an increase in leaf biomass production, root dry weight and spongy intercellular space, and as a decrease in leaf loss, yellowing of leaves and palisade layer thickness. The results suggest that sufficient nitrogen supply can confer the birch with greater resistance to ozone.