Ozone-induced visible injury to plants is relatively common in North America but has rarely been reported in Europe. The south of Switzerland (canton Ticino) has been identified as having high ambient ozone exposures relative to other parts of Switzerland, with accumulated annual exposures over a threshold of 40 ppb (40 nl O₃ litre⁻¹) for daylight hours from April to September of >30 ppm×h. Ozone-induced foliar injury on black cherry (Prunus serotina) has been observed in the area. The main purpose of this study was to establish whether seedlings of black cherry grown in three different treatments (open plots [ambient ozone concentrations], chambers receiving non-filtered [96% ambient] air and charcoal-filtered chambers [ca. 60% ambient]) showed significant differences with respect to the various aspects of ozone injury development, particularly trends in injury development, the survival probabilities, the average injury level of a tree and the proportion of trees showing any sign of injury over the summer growing season. Visual estimates of the percentage of leaves showing symptoms (adaxial stipple and leaf reddening) on a plant and the percentage of leaf area of only the injured leaves which showed symptoms of ozone injury were recorded weekly for each seedling for eight consecutive weeks. These two scores were multiplied to derive a measure of injury (Y) which can be interpreted as the proportion of injured leaf area on a tree. This derived score was used in all subsequent analyses. In this paper we assess whether, overall, an increasing trend in the injury levels, as given by the values of Y, was evident. We have also estimated the probability F(t) that the first sign of injury (measured by Y) might occur after a given number t of weeks. The smaller the value of F(t) for a given t, the stronger the effect of the treatment (i.e. the treatment causes relatively more damage). The average injury level of a tree and the proportion of trees that showed any sign of injury over the 8-week period were also analysed. The results from the open plots, the non-filtered chambers and the charcoal-filtered chambers were significantly different. However, no statistically significant difference was found between the open plots and the non-filtered chambers.