To improve the management of ageing urban trees, the role of wood-decay fungi as potential causes of stem breakage was investigated among hazardous trees removed in the Helsinki City area during [2001][2002][2003][2004]. The study material comprised 194 trees, and included 76 Tilia spp. trees, 58 Betula spp. and 60 Acer spp. Thirteen species or genera of commonly occurring decay fungi were identified on the basis of fruiting bodies and pure cultures. The occurrence of the fungi was investigated in terms of frequency, visibility and as potential causes for stem breakage. Most hazardous fungi caused extensive horizontal decay in the stem; such fungi were Ganoderma lipsiense on Tilia and Acer, Phellinus igniarius on Acer, Inonotus obliquus and Cerrena unicolor on Betula and Kretzschmaria deusta on Acer, Tilia and Betula. Typically, Rigidoporus populinus was frequently present in weak fork formations on Acer trees. Agaric fungi (Pholiota, Armillaria, Pleurotus and Hypholoma) were frequently recorded but were of minor importance from the point of view of tree breakage hazard.