Site-specific estimates for various environmental stress factors were related with measured crown condition data at a systematic 16 x: 16 km(2) grid over Europe, according to previously stated hypotheses, using a multiple regression approach, including interactions, and lagged effects of stress factors. Methodological differences among countries accounted for > 30% of the variation in defoliation. Nevertheless, crown condition was found to vary naturally with tree age, altitude, drought stress and, most likely, also pathogenic fungi and insects. Significant impacts of air pollution (specifically ozone but also NOx, SOx and acid deposition) were found at regional levels in parts of central Europe, particularly for deciduous species. Impacts seemed less significant for conifers, especially for spruce, but this might be affected by confounding effects or strong correlations between (a harsh) climate and (low) atmospheric deposition in the area where spruce predominates. National studies indicate that ozone and acid deposition can have a significant effect on the defoliation of spruce as well. We conclude that while forest condition varies naturally, continued emissions will contribute further to forest decline in the long term.