Three-year-old clonal saplings of Picea abies L. Karst. were fumigated for 60 d during the summer of 1985 with four different levels of O3. Visible injury (in the form of severe, uniform brown necrosis and shedding of affected needles) occurred after a frost in November on the older needles of three of the clones which had received over 200 μg m−3 of O3 during the summer, 47 d previously. No visible injury occurred during the fumigation period or on the current (1985) year needles. It is suggested that the results provide preliminary evidence for an enhancement of frost sensitivity by O3 which may be significant in the current wave of forest decline affecting high-altitude forests in central Europe.