The aim of this work was the risk assessment of genetically engineered apple trees in the case of a potential release. One part of the investigations concerns the relative frequency of gene flow from transgenic to non-transgenic apple cultivars in the environment. Another objective deals with the stability of integration and long-term expression of transgenes in vegetatively propagated in vitro plants and after grafting of ex vitro plants in different scion/rootstock combinations. Furthermore the transport of transgene products in grafted ex vitro plants was analysed.