Wood conditions and sound wood thickness were determined with Resistograph F500 in living trees of 2 conifers (silver fir, Norway spruce) and some broadleaved tree species with diffuse porous wood (beech, horse chestnut and lime). Samples were obtained from felled urban trees of the City of Ljubljana and from freshly cut trees from a log storage place. Resistograph F500 detected occurrence of early and late wood, compression wood, ring shakes, reaction zones, barrier zones, wound-wood and decayed wood in all the investigated species. Occurrence of the bark, discoloured wood and dehydration was not detected in all of the investigated trees. Data obtained using Resistograph F500 are helpful in detection of occurrence and determination of the extent of defects in living trees. The device should be used by educated and experienced persons to avoid misinterpretations and consequently wrong decisions.