Saplings of an ozone sensitive clone of birch (Betula pendula Roth, KL-5-M) were well-watered or exposed to mild drought-stress combined with ambient or elevated (1·5 x the ambient) ozone for 11 weeks in open-field conditions in central Finland. Stomatal response, visible injury, chlorophyll and nutrient content, and changes in cellular anatomy and plant growth were studied. Drought stress alone, in ambient ozone, reduced stomatal density and stomatal conductance. Drought stress and ozone effects were additive, reducing total leaf number, foliage area and starch formation in mesophyll cells. Drought stress and ozone effects were additive, increasing the N concentration in the leaves, the thickness of the upper epidermal cell wall, the number of pectinaceous projections of mesophyll cell walls, and the vacuolar tannin-like depositions and phenolic droplets, regarded as signs of activated stress defence mechanisms. The increase in specific foliage mass, cytoplasmic lipids (younger leaves), and a condensed appearance of the upper epidermal mucilaginous layer were caused by both drought and ozone, but were not additive. The results show that combined drought stress contributed to birch responses to 1·5 x current ambient ozone concentrations, corresponding to critical-level ozone exposure. The only beneficial effect of drought stress was the slight reduction of visible leaf symptoms induced by ozone in autumnal leaves.