The warming of the atmosphere observed during the last century is supposed to have intensified the hydrological cycle. As a consequence the occurrence of drought is expected to have changed in the last decades. To detect trends regarding drought in forested ecosystems numerous time series of a newly developed ecophysiologically based drought index are analysed. The time series are simulated by the hydrological model WAWAHAMO (WAldWAsserHAushaltsModell, German, i.e. forest water balance model) for the observation period from 1969 to 1998 covering the entire forested area of Switzerland with a horizontal resolution of 1km. Furthermore, time series of related variables are examined for trends, i.e. observations of annual mean temperature, annual and seasonal precipitation totals, and simulations of annual transpiration totals and the length of the vegetation period. Over large parts of the forested area of Switzerland the analysis detected an increase of drought during the observation period. The trend is particularly pronounced in rather dry areas as the inner-alpine valley of Valais or near the northern border towards Germany. Hardly any trend can be detected in the prealpine regions, which are characterised by a huge surplus of water resulting from strong and frequent orographically forced precipitation events. Moreover, there is a definite increase in temperature, transpiration and the length of the vegetation period. No systematic trend could be detected for precipitation, neither for annual nor for seasonal totals.