Natural variations in the oxygen isotope ratio 18O/16O are occurring in the hydrological cycle as a result of isotope fractionations during evaporation and condensation. These processes imprint a valuable climatic signal in the precipitation, which is stored in ice caps as well as in the cellulose of trees. Recent developments in the continuous-flow analysis of 18O/16O of organic matter now enable a systematic application of this method in tree rings. It becomes possible to build maps of the past oxygen isotope distribution in continental areas, yielding important information on regional climate changes. In this paper, the factors influencing the isotope composition of tree rings are discussed with an example from trees in northern Eurasia. Oxygen isotope values of Larix, Picea and Pinus trees were measured over a large climatic gradient extending from Norway to Siberia. The spatial isotope variations were highly correlated to the annual mean temperature (r2 = 0.84), whereby the slope of the corresponding regression line was 0.35%/degrees C. When considering the changes in 18O/16O during the 20th century, not only the temperature, but also changes in the precipitation patterns have to be considered, in particular the observed increase in the amount of winter precipitation.