The health of Swiss forests is now being monitored at some 160 permanent sites at 8 km intervals on a square grid. Data on the conditions of the tree crowns in 1993 and on the soil, climate, and topography have been analysed to determine the extent to which crown condition depends on these factors. Transparency was the only widespread symptom of ill health or lack of vigour, and it was common in only three species, Picea abies (L.) Karst., Abies alba Mill., and Fagus sylvatica L. Relations between transparency and the environmental variables recorded were weak, and for Fagus too weak to be of consequence. The strongest relations are with the soil water holding capacity for Picea, and with water holding capacity and carbonate content for Abies. It seems that the loss of leaves is the trees' natural physiological response to lack of water in the soil: drought is a sufficient explanation for the observed crown condition. The results and interpretation accord with those elsewhere in Europe in 1993.