The purpose of this experiment was to investigate the effects of two potassium fertilization treatments on fruit set and flower bud, flower and bract leaf nutrient concentrations in Prunus cerasus 'Stevnsbaer' on Prunus avium and 'Colt' rootstocks. Single applications of KNO3 or KCl were applied in June or August, respectively, during the 2001 growing season. Leaf samples were collected for nutrient analyses in August just before harvest and at the beginning of October. Samples of flower buds, flowers and bracts were also collected the following spring (2002). Tissue for nutrient analysis was ashed and analysed using inductively coupled plasma mass spectrometry. Fruit set percentage was recorded on spurs and extension shoots in 2002. The nutrient analyses showed that control trees on Prunus avium had a higher concentration of leaf potassium compared with control trees on 'Colt'. Leaf potassium concentrations were higher in KNO3 treated trees compared to the control trees at both sampling times and on both rootstocks. The late application of KCl had no effect on the potassium concentration but it increased the level of leaf chloride concentration in trees on both rootstocks in October. The nutrient analyses of flower buds, flowers and bracts showed some significant differences in the nutrient concentrations as an effect of both rootstocks and treatments. Fruit set on both spurs and extension shoots was significantly affected by rootstock whereas the fertilization treatment had no effect. Prunus avium had the highest percentage of fruit set on both spurs and extension shoots.