The paper contains a comprehensive analysis of the variability of density in middle-sized specimens of silver birch (Betula pendula Roth). Wood density tests were carried out using transverse and longitudinal sections of five middle-sized silver birch logs harvested from the Czarna Białostocka Forest District. Selected wood characteristics, on which the density depends, i.e. the proportion of young and mature timber and the impact of defects, were investigated. Wood density testing consisted of a non-destructive method, where a MGD-05 densimeter and X-ray tests were used. The test findings were statistically analysed. They showed that a mean density of B. pendula timber depends on the position on the longitudinal and transverse sections of the trunk. Its variation is not influenced by geographical direction (north-south). Middle-sized silver birch wood is practically entirely composed of juvenile wood. The average density is much higher than the mean density at breast height level, and when measured at ¼, the difference has not statistical significance compared with the mean density of the entire tree log.