Mature apple (Malm domestica Borkh.) trees were exposed to episodes of 100 and 150 nl l−1 ozone in 1990 and 1991 respectively, in open-top chambers at the University of Nottingham, Sutton Bonington Campus, UK. The episodes were for 8 h daily and for not more than 3 d in succession, with a seasonal total of 37 d in each year. Leaf fall was monitored by counting leaves on marked shoots at frequent intervals. A significant interaction between ozone exposure and leaf age was observed: exposure to ozone accelerated autumn leaf fall in old leaves. The relationships between experimental conditions, leaf age and plant material are discussed, and the implications for UK flora are considered.