Biomass and biomass expansion factor functions are important in wood resource assessment, especially with regards to bioenergy feedstocks and carbon pools. We sampled 48 poplar trees in seven stands with the purpose of estimating allometric models for predicting biomass of individual tree components, stem-to-aboveground biomass expansion factors (BEF) and stem basic densities of the OP42 hybrid poplar clone in southern Scandinavia. Stand age ranged from 3 to 31 years, individual tree diameter at breast height (dbh) from 1.2 to 41 cm and aboveground tree biomass from 0.39 to 670 kg. Models for predicting total aboveground leafless, stem and branch biomass included dbh and tree height as predictor variables and explained more than 97 % of the total variation. The BEF was approaching 2.0 for the smallest trees but declined with increasing tree size and stabilized around 1.2 for trees with dbh >10 cm. Average stem basic density was more than 400 kg m−3 for the smallest trees but declined with increasing tree height and stabilized around 355 kg m−3, at a tree height of about 20 m. Existing biomass functions from the literature all underestimated the measured sample tree biomass. Possible explanations include not only differences in competition among trees in the examined stands and site conditions but also differences in sampling procedures. We observed that basic density increased with height above the ground. This trend may have led to the observed underestimation by existing biomass functions including only few samples from the lower end of the stems.