Within the framework of a pan-European evaluation of the forest condition data, the available crown condition data (defoliation) have been correlated with site-specific estimations of various stress factors, both natural and anthropogenic. The effect parameters include the actual defoliation and the change and trend in defoliation for six important tree species. The predictor variables (stress factors) include available and interpolated soil data, interpolated and modelled meteorological data and modelled air pollution and deposition data. Various threshold values for anthropogenic stress factors were included, including site-specific estimations for the critical deposition levels. The variation in the actual defoliation is mainly correlated with the different reference trees in the different countries. Besides, age is an important factor, followed by different natural and anthropogenic stress factors, depending on the tree species. The changes and trends in defoliation did not show a clear correlation with the considered stress factors.