The effects of long-term elevated UV-B radiation on silver birch (Betula pendula Roth) seedlings were studied over three growing seasons in an outdoor experiment in Finland started 64 days after germination. One group of seedlings was exposed to a constant 50% increase in UV-BCIE radiation, which corresponds to 20–25% of ozone depletion; another group received a small increase in UV-A radiation and a third (the control group) received ambient solar radiation. Changes in growth appeared during the third growing season; the stems of the UV-B treated seedlings were thinner and their height tended to be shorter compared with that of the control seedlings. In contrast, there were no UV-B effects on biomass, bud burst, bud dry weights, leaf area, rust frequency index or chlorophyll concentrations in any of the summers. During the three-year study, the flavonols were significantly increased by the elevated UV-B only in the first growing season. The responses varied greatly among individual compounds; the most induced were the quercetin glycosides, while the main flavonols, myricetins, were reduced by the UV-A control treatment. In the second summer phenolic acids, such as 3,4′-dihydroxypropiophenone-3-glucoside, neochlorogenic acid and 5-coumarylquinic acid, were increased by the UV-B treatment. In the third year, the constitutive concentrations of phenolics were not affected by the UV-B treatment.