Studies on wood basic density (BD) vertical variations become essential to predict more accurately the within-stem distributions of biomass and wood quality in the forest resource. The vertical variation of wood BD in the stem has been little studied until now, most BD studies being based on measurements taken at breast height. The main objective of this work was to observe and to understand the patterns of vertical BD variation within stems in relation to classical dendrometric variables and to propose relevant equation forms for future modelling. Two softwood species were studied: Abies alba and Pseudotsuga menziesii. Contrasted thinning intensities were studied including strongly thinned plots versus control plots without thinning. BD was most of the time highest at the base of the tree for both species. Then, after a strong decrease from the base of the tree, an increase in BD was often observed towards the top of the tree especially for A. alba. The variation in BD with height was stronger for the unthinned plots than for the heavily thinned ones of A. alba. The opposite was observed for Ps. menziesii. The modulation of growth rate and tree size through thinning intensities modifies the observed vertical variations in BD. Two types of biexponential models were proposed to describe BD variations. The first model used the height in the stem and classical easily-measurable tree variables as inputs, the other one additionally used BD at breast height (BD130). The relative RMSE of BD for A. alba and Ps. menziesii were 9.9% and 8.1%, respectively, with the model without BD130 and 7.6% and 5.9%, respectively, with the model including BD130.